Order–Disorder Transition in Kesterite Cu₂ZnSnS₄: Thermopower Enhancement via Electronic Band Structure Modification

Abstract

The order−disorder transition of kesterite (CZTS, Cu 2 ZnSnS 4 ) from I4̅ to I4̅ 2m crystal structures has a marked effect on Seebeck coefficient, which displays a sharp enhancement at the transition temperature, around 533 K. Considered to be detrimental for the performance of photovoltaic kesterite, the order−disorder transition appears to be beneficial for thermopower. Experimental data and ab initio calculations explain the origin of this enhancement: the increase of crystal symmetry in the disordered polymorph leads to a favorable electronic band structure characterized by flat and converged bands. At the transition, a sharp drop in mobility and increase in carrier concentration experimentally prove this mechanism of Seebeck enhancement. This, other than providing a new understanding of the material, can cast light on some profitable mechanisms to enhance the thermoelectric performance. Additionally, the increase in Seebeck provides an efficient tool to observe the transition and possibly to quantify disorder.

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

“…5(a) and 5(b)] do not match with the experimental results. This is expected as the GGA of the exchange-correlation functional is known to strongly underestimate the band gap [8,39]. Although the DFT representation in Fig.…”

“…Indeed, above 533 K the ordered tetragonal polymorph evolves into a disordered tetragonal [space group I -42m, Fig. 1(b)] through the reversible order-disorder phase transition [7,8,30,31], involving a mixed and random occupation of Cu and Zn cations in the 4d Wyckoff positions (whereas in the ordered tetragonal the 2c site is occupied by Cu and the 2d by Zn) [7,30].…”

“…Alternatively, behaviors of this kind often relate to the mechanism of band degeneracy (the tendency of electronic bands to converge within an energy range of approximately 2 k B T), that can selectively promote the thermopower without penalizing carrier conduction. [4,[6][7][8]35,44] However, band convergence does not seem to play a significant role in this case, as little difference is observed in the spacing of bands between the two structures [Figs. 5(a) and 5(b)], with the top three bands being separated by more than 2 k B T. We therefore put forward that the midgap states exert a strong influence in decoupling electrical conductivity and the Seebeck coefficient.…”

“…This is usually expressed by the TE figure of merit, zT = σ S 2 T/k, which combines three fundamental but conflicting characteristics: electrical conductivity (σ ), Seebeck coefficient (S), and thermal conductivity (k), while T is the absolute temperature. TE research has recently led to a number of methods to improve zT: nanostructuring [1][2][3] reduces k L (lattice component of k) without excessively penalizing electrical transport; chemical doping improves carrier concentration and conduction, though * eleonora.isotta@unitn.it † binayak.mukherjee@unitn.it ‡ paolo.scardi@unitn.it often at the expense of the Seebeck coefficient; crystal and electronic structure engineering [4][5][6][7][8] mainly enhance the Seebeck coefficient. Many efforts have been directed at promoting a phonon-glass electron-crystal behavior, which aims at increasing the electrical conductivity and suppressing the thermal conductivity [9][10][11].…”

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.…”