Origin of a Simultaneous Suppression of Thermal Conductivity and Increase of Electrical Conductivity and Seebeck Coefficient in Disordered Cubic Cu ₂ ZnSnS ₄

Abstract

The parameters governing the thermoelectric efficiency of a material, Seebeck coefficient, electrical, and thermal conductivities, are correlated and their reciprocal interdependence typically prevents a simultaneous optimization. Here, we present the case of disordered cubic kesterite Cu 2 ZnSnS 4 , a phase stabilized by structural disorder at low temperature. With respect to the ordered form, the introduction of disorder improves the three thermoelectric parameters at the same time. The origin of this peculiar behavior lies in the localization of some Sn lone pair electrons, leading to "rattling" Sn ions. On one hand, these rattlers remarkably suppress thermal conductivity, dissipating lattice energy via optical phonons located below 1.5 THz; on the other, they form electron-deficient Sn-S bonds leading to a p-type dopinglike effect and highly localized acceptor levels, simultaneously enhancing electrical conductivity and the Seebeck coefficient. This phenomenon leads to a 3 times reduced thermal conductivity and doubling of both electrical conductivity and the Seebeck coefficient, resulting in a more than 20 times increase in figure of merit, although still moderate in absolute terms.

Topological Anderson Insulator in Cation-Disordered Cu2ZnSnS4

“…CZTS is a quaternary chalcogenide compound extensively investigated for its potential applications, primarily in photovoltaics [ 36 , 37 , 38 , 39 , 40 , 41 ], and recently thermoelectrics [ 28 , 42 , 43 , 44 , 45 , 46 , 47 ]. The most ubiquitous polymorph of CZTS is the kesterite structure ( Figure 1 a), which crystallizes in the tetragonal I-4 space group.…”

“…This disorder induces a narrowing of the band gap compared to the ordered tetragonal polymorph ( Figure 1 d,e), while the increase in global symmetry introduces a three-fold degeneracy at the center of the irreducible Brillouin zone (Γ-point) in the valence band maximum ( Figure 1 e). In a recent article, Isotta et al [ 28 ] have presented the synthesis of a novel polymorph of CZTS, this time with full cation disorder. This is manifested as a complete randomization of atoms in the cation position ( Figure 1 c).…”

“…These robust surface states are expected to support quasi-metallic surface transport. This would contribute significantly to the improved conductivity observed in disordered cubic CZTS [ 28 ].…”

“…However, it is known that CZTS crystallizes in multiple polymorphs. In a recent study, Isotta et al [ 28 ] demonstrated remarkably improved thermoelectric properties in a cubic polymorph of CZTS with complete cation disorder. This polymorph, which was synthesized using high-energy reactive mechanical alloying (ball-milling), shows a simultaneous improvement in both Seebeck coefficient and electrical conductivity, as well as a lower thermal conductivity compared to the ordered tetragonal polymorph.…”

An Overview of the Strategies for Tin Selenide Advancement in Thermoelectric Application

“…The individual and synergetic methods are of great importance to boost the electrical and thermal properties of TE materials [160]. Table 5 summarizes the synthesis methods and approaches used to simultaneously increase power factors and reduce thermal conductivity in SnSe materials.…”