Thermoelectric Performance of Sb₂Te₃-Based Alloys is Improved by Introducing PN Junctions

Abstract

Interface engineering has been demonstrated to be an effective strategy for enhancing the thermoelectric (TE) performance of materials. However, a very typical interface in semiconductors, that is, the PN junction (PNJ), is scarcely adopted by the thermoelectrical community because of the coexistence of holes and electrons. Interestingly, our explorative results provide a definitively positive case that appropriate PNJs are able to enhance the TE performance of p-type SbTe-based alloys. Specifically, owing to the formation of the charge-depletion layer and built-in electric field, the carrier concentration and transport can be optimized and thus the power factor is improved and the electronic thermal conductivity is decreased. Meanwhile, PNJs provide scattering centers for phonons, leading to a reduced lattice thermal conductivity. Consequently, the p-type (BiTe)-(SbTe) composites comprising PNJs achieve a ∼131% improvement of the ZT value compared with the pure SbTe. The increased ZT demonstrates the feasibility of improving the TE properties by introducing PNJs, which will open a new and effective avenue for designing TE alloys with high performance.

Current State‐of‐the‐Art in the Interface/Surface Modification of Thermoelectric Materials

“…Due to the different n‐ and p‐type nature of semiconductor materials, PN junctions at the interfaces can be formed when different matrix types are dispersed in a semiconductor material. According to the schematic [ 129 ] of a PN junction illustrated in Figure 11 a and 11b, two kinds of majority carriers exist in different n‐ and p‐type materials, and the built‐in electric field will be established after voltage is applied. The band alignment in the interface will yield an ultrahigh interface potential, allowing the carrier transport to occur only along the homogeneous junctions.…”

“…In principle, the reduced n in composites with PN junctions is mainly attributed to the combination of holes and electrons during the formation of the carrier‐depletion layers. [ 129 ] The decreased μ originates from the forbidden transport of electrons and holes by the built‐in electric field. This decrease in carrier concentration and Hall mobility will improve the electrical properties as a result.…”

“…The powders of Bi 2 Te 3 and Sb 2 Te 3 were separately synthesized and mixed according to the stoichiometry of (Bi 2 Te 3 ) x (Sb 2 Te 3 ) 1− x . [ 129 ] The energy‐dispersive X‐ray spectroscopy (EDS) line‐scanning analysis of the Sb/Bi elemental distribution at the grain boundaries revealed that both constituents remained chemically intact and no solid solution was formed during the sintering process. Furthermore, compared with the Sb 2 Te 3 pristine powder, the carrier concentration and Hall mobility significantly decreased (Figure 11c).…”

“…a) Distribution of carriers prior to the PN junction formation; b) Band alignment at the p‐type Sb 2 Te 3 and n‐type Bi 2 Te 3 interface; c) Dispersion‐content dependence of n / n 0 , µ / µ 0 , and zT 0 / zT at 308 K (Subscripts with zero stand for pristine materials). [ 129 ] Reproduced with permission. [ 129 ] Copyright 2018, American Chemical Society.…”

Temperature Driven Unusual Reversible p‐ to n‐Type Conduction Switching in Bi2Te2.7Se0.3

“…Another interesting aspect of p–n transition in the present case is gradual lowering of Seebeck coefficient with increasing temperature from positive value at 300 K to zero at 487K and negative value at high temperature. This can be explained by the contributions of both p‐ and n‐type charge carriers in the electrical transport hence total Seebeck coefficient can be expressed as . Here α p and α n are the contributions of holes and electrons to the total Seebeck coefficient.…”