Nanoscale defect structures advancing high performance n-type PbSe thermoelectrics

Highlights

• Recent advances on high performance n-type PbSe thermoelectrics are discussed.

• Designing defect structures to enhance thermoelectric performance are discussed.

• The effects of nanoscale defect structures on charge and thermal transport properties are introduced.

Abstract

This short review discusses recent advances in n-type PbSe-based bulk thermoelectric materials. Their performance has rapidly progressed recently and emerges as one of most efficient n-type thermoelectrics in the mid-temperature range (500–800 K), rivaling PbTe-based materials. This success is mainly attributed to the development of new performance-enhancing strategies employing nanoscale defect structures. We introduce intrinsic thermoelectric properties of PbSe based on crystal and electronic band structures. We then discuss how nanoscale defect structures can modulate charge and phonon transport properties and serve as a key to achieving higher ZT in n-type PbSe with the examples of current state-of-the-art systems.

Introduction

Thermoelectric modules are solid-state electronic devices constructed by joints of n- and p-type semiconductors with proper electrical and thermal transport properties [1], [2], [3]. A temperature gradient applied to the device renders mobile charge carriers to move from the hot to the cold end, consequently generating an electric potential spontaneously. They can produce a usable form of electric energy by harvesting waste heat from various sources. Note that electricity is mostly generated by fossil fuels. This conversion process is unavoidably accompanied by dissipating waste heat, which is approximately 66% of overall input from primary energy sources [4]. In this regard, thermoelectric technology can contribute to solving both environmental and energy crisis that human faces [4], [5]. By virtue of all-solid-state electronic device structure with no working fluid, thermoelectric power generators operate without the release of undesirable chemical residues, noise, and vibration, thereby mechanically reliable and environmentally benign [5], [6]. Their power conversion efficiency is directly determined by the performance of thermoelectric materials. The latter is commonly described by the dimensionless figure of merit, ZT = S²σT/κ_tot [7], [8], [9], where S is the Seebeck coefficient, σ is the electrical conductivity, their product S²σ is the power factor (PF), T is the absolute temperature, and κ_tot is the total thermal conductivity contributed from charge carriers (κ_ele) and lattice vibration (κ_lat) [10].

Note that approximately 90% of all waste heat in the USA arises from intermediate temperature (500 ≤ T ≤ 900 K) heat sources [11]. Both n- and p- type PbTe-based materials with rock-salt structure have been the most extensively studied thermoelectric system operating in such temperatures for the past several decades [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. They show intrinsically low κ_lat mainly due to innate giant anharmonic phonon scattering [25] and large Seebeck coefficient arising from energetically favorable electronic band structures when properly doped [26]. Their thermoelectric performance has been further advanced by many innovative strategies such as nanostructuring [27], hierarchical architecture [18], and artificially modulating band structures [17], [28]. The maximum ZT (ZT_max) for n- and p-type PbTe is exceptionally high at ~ 1.8 at 773 K [29] and 2.5 at 923 K [19], respectively, which is among the highest for polycrystalline materials. However, their broad commercial applications are largely impeded by high cost of Te. Natural abundance of Te is only 0.001 ppm [30], similar to that of platinum. On that account, the widely sought-after goal is developing less expensive alternatives to PbTe.

PbSe can be a promising candidate to replace PbTe given its similar crystal, electronic [31], and phonon structures [32] as well as 50-fold richer abundance of Se than Te in the Earth-crust [30]. The former enables one to adaptively apply many innovative performance-enhancing strategies developed for PbTe to PbSe system. For example, nanostructuring to reduce thermal conductivity and band engineering to improve power factor are also effective for PbSe as PbTe thermoelectrics. PbSe intrinsically exhibits κ_lat as low as PbTe at high temperatures, which are the optimal operation range of this material for thermoelectric power generation. Advantageously, it melts much higher than PbTe, providing better thermal stability and wider and higher operation temperature range. Nonetheless, PbSe thermoelectrics has been underdeveloped and has much underperformed PbTe. Very recently, several research efforts artificially stabilized multiscale defects such as atomic and nanoscale structures in the PbSe bulk matrix [33], [34], [35], [36], [37], [38], [39], significantly influencing on its electrical and thermal transport properties. As a result, especially n-type PbSe performs comparable to the corresponding PbTe and other state-of-the-art n-type thermoelectric materials (Fig. 1a and b), thereby emerging as the most efficient thermoelectric system in the intermediate temperature regime [29], [40], [41], [42], [43].

In this article, we review cutting-edge advances in n-type PbSe thermoelectric system focusing on new performance-enhancing strategies. First, we discuss electronic and phonon band structures of PbSe to give fundamental understandings for the potential and limitation of n- and p-type PbSe thermoelectrics. Second, we introduce the important recent progresses in n-type PbSe, which mainly employ various lattice defects for high thermoelectric performances. This section provides the subdivisions according to the dimensionality of defects from point defects to dislocations and nanoscale precipitates. Finally, we outlook future directions for enhancing thermoelectric performance of PbSe, shedding light on the development of cost-effective high performance thermoelectric materials.