**4. Defects in 2D bulk materials for thermoelectric power generation**

Thermoelectric (TE) materials have the potential to reduce global energy crisis and global warming effects by converting waste heat to electricity. As of 2005, the world energy usage was ~15 terawatts of energy, of which ~90 % was first converted to heat and the remainder ~10 % of energy was utilized [24]. In general, power plants and the transportation industry are the two main sources of heat energy losses that contribute to global warming. In recent years, prototype car models developed by automobile industries BMW and Ford have successfully transformed the waste heat from car exhausts to electricity using thermoelectric power generators, thus improving the fuel efficiency [24, 34].

A basic thermoelectric energy conversion module consists of *n* and *p*-type semiconducting materials, connected electrically in series and thermally in parallel [35]. The maximum thermoelectric efficiency is a product of the Carnot Efficiency and a term consisting of *ZT* or the thermoelectric figure of merit, which embodies interrelated material parameters, as given by *ZT* = *α<sup>2</sup> σT*/*k*, where *α* is Seebeck coefficient or thermopower, *σ* is the electrical conductivity, and *k*(= *kE* + *kL*) is the total thermal conductivity comprised of electronic (*kE*) and lattice contributions (*kL*), respectively. The main challenge of improving the energy conversion efficiency and consequently the *ZT* of thermoelectric materials is the inherent *coupling* between the electrical conductivity and the Seebeck coefficient. In recent years, there has been signifi‐ cant scientific progress in the field of thermoelectrics with the use of nanostructured materials (e.g., superlattices, nanowires, and nanocomposites) that have simultaneously increased the power factor (the numerator of *ZT*) and reduced the thermal conductivity to achieve a high *ZT* > 1 [36].

#### **4.1. Quantum confinement effects in 2D thermoelectric materials**

In the early 1990s, Hicks et al. [37] predicted intriguing changes in transport properties upon lowering the dimensionality of existing bulk materials (e.g., from 3D to 2D) that were not observed in the corresponding bulk materials. A dramatic increase in the density of states (DOS) of low-dimensional materials was predicted that could increase the Seebeck coefficient and potentially decouple the electronic transport properties. Moreover, the presence of numerous interfaces in low-dimensional materials also increased phonon scattering effects that reduced the lattice thermal conductivity, thus introducing opportunities to independently vary all the parameters constituting the *ZT* [38]. In the low-dimensional thermoelectric materials, dramatic changes in the density of states were observed in the PbTe-based thermo‐ electric materials owing to the quantum confinement effects that led to further *decoupling* of the TE transport properties and an enhancement of *ZT* [39].

Nevertheless, it is challenging to fabricate low-dimensional materials for commercial thermo‐ electric applications and devices, which must be a cost-effective and facile process. In addition, the nanostructured thermoelectric materials have to be thermodynamically stable to retain the desired 2D properties over time, to make the devices reliable and long lasting. To be able to make use of the advantages of low-dimensional materials as well as robustness of the bulk materials, bulk nanomaterials or nanocomposites have been used to enhance the thermoelec‐ tric performance of existing thermoelectric materials such as SiGe and PbTe [40, 41].

Controlling the multi-scale microstructures via defect engineering and consequently the length scales of the electrical and thermal transport is essential for enhancing TE performance. However, material properties (thermopower, electrical, and thermal conductivity) dictating the ultimate compatibility factor and *ZT* are inherently coupled at the nanoscale. Thus, any efforts to improve only one particular property (e.g., thermopower) using microstructure changes are often futile as they inadvertently deteriorate remaining properties (e.g., electrical conductivity). In this section, we describe how charged defects in 2D materials can introduce low-energy carrier filtering and selective charge carrier scattering (e.g., hole excessively scattered compared to electrons) to improve *ZT* and compatibility factor in Bi2Te3.
