Maximizing the thermoelectric performance of topological insulator Bi₂Te₃films in the few-quintuple layer regime

Abstract

Using first-principles calculations and Boltzmann theory, we explore the feasibility to maximize the thermoelectric figure of merit (ZT) of topological insulator Bi 2 Te 3 films in the few-quintuple layer regime. We discover that the delicate competitions between the surface and bulk contributions, coupled with the overall quantum size effects, lead to a novel and generic non-monotonous dependence of ZT on the film thickness. In particular, when the system crosses into the topologically non-trivial regime upon increasing the film thickness, the much longer surface relaxation time associated with the robust nature of the topological surface states results in a maximal ZT value, which can be further optimized to ~2.0 under physically realistic conditions. We also reveal the appealing potential of bridging the long-standing ZT asymmetry of p-and n-type Bi 2 Te 3 systems.

Topological insulators for thermoelectrics

“…Thus, the conductance is used instead of the conductivity for properly describing the TE size effect. 28,29 Based on the multiple-channel model, theoretical studies have been performed under two different conditions, with and without the hybridization effect in boundary states.…”

“…As a result, by controlling the geometric sizes and introducing disorders and defects, the relative contributions of electrons (including both bulk and boundary states) and phonons can be tuned and TE properties of TIs thus can be improved. 28,29 The schematic band structure of TIs is depicted in Fig. 2a.…”

“…26,27 By introducing different levels of disorders or defects and by controlling the position of Fermi level, it is expected that TE property can be greatly tuned and optimized. 28,29 The underlying mechanism is that electronic transport is immune to backscattering by non-magnetic disorders and defects, while phonon transport is not. Thus, the two kinds of transport can be effectively decoupled, realizing the "phononglass, electron-crystal" concept and improved TE performance in TIs.…”

“…Thus, the two kinds of transport can be effectively decoupled, realizing the "phononglass, electron-crystal" concept and improved TE performance in TIs. 24,[27][28][29] As the thickness of TI films or the radius of TI nanowires decreases to the order of the localization width of boundary states, a hybridization gap opens at the DP. [30][31][32] The appearance of a hybridization gap is advantageous for the Seebeck effect but partly destroys the topological protection of the boundary states.…”

“…[30][31][32] The appearance of a hybridization gap is advantageous for the Seebeck effect but partly destroys the topological protection of the boundary states. 28,29 Nevertheless, the TI boundary states can still have superior transport ability even with a hybridization gap, leading to an enhanced ZT. 33 The theoretical studies under the two conditions (i.e., without and with the hybridization effect) will mainly focus on discussing electronic transport, as factors that affect thermal properties, such as increased boundary scatterings, should be similar as that for other common nanostructures, which has been well established in previous classical articles.…”