Challenges and Novel Strategies in High-Performance Thermoelectric Material Engineering

Thermoelectric (TE) materials have been recognized as promising materials and make a prime contribution to converting waste heat into valuable electrical energy without further loss. In recent decades, researchers have witnessed the development of new and efficient mechanisms/ strategies to optimize the figure of merit (<i>ZT</i>) of these materials to recover low-grade heat more efficiently. The TE materials and devices are of greater importance because of their small size (scalable), lack of moving parts, high reliability, feasibility for miniaturization, lack of noise and pollution, and lack of emission of greenhouse gasses. An application of such characteristics delivers exclusive TE technology for numerous fields or niche applications, such as sensing and wearable electronics, waste low-quality heat recovery, TE refrigeration, aerospace, cogeneration, medical thermostats, zero radiation, and solid-state operation. To fabricate high-performance TE materials, a balanced trade-off between TE parameters is indispensable to optimizing the <i>ZT</i> value and is the key challenge to engineering such thermoelectrics. To achieve the highest <i>ZT </i>value, the maximization of the TE power and the electrical conductivity, and the minimization of the thermal conductivity are essential. To some extent, the hit-and-trial method can be effective in optimizing TE properties. Practically, it is possible to enhance the power factor by aligning and converging the electronic bands, modifying the electronic structure, and creating resonance states. All-scale hierarchical scattering and entropy engineering incorporate all of these outcomes into a single material system. Band flattening and resonant levels are two commonly adopted ways to increase the effective mass which has a significant contribution to optimizing TE performance. The first one can be brought about via the endorsement of dopants accommodating highly localized orbitals which leads to a fall in the overlapped orbitals. The Kane-band model illustrates that band flattening can enhance the bandgap <i>E_g</i> through band dispersion. And in the latter approach, the excess density of states (DOS) can be achieved via electronic coupling and is more efficient at a lower temperature than at a higher temperature because of the variation in relaxation time.<br/>Low dimensionality and electronic structural modifications, such as electronic band structure engineering and the creation of extrinsic defects through alloying and nanostructuring to suppress the lattice thermal conductivity (<i>k_l</i>), are efficient strategies to enhance the <i>ZT </i>of TE materials. The confinement of the electrons in low dimensions (D) such as 1D or 2D can improve the performance of TE materials by abruptly altering the electronic DOS. Low-dimensionality strategy is a milestone to initiate the nanostructure (quantum-well, superlattice structures, nanowires) studies in TE materials. When the phonon has a longer mean free path (MFP) than the electron then nanostructuring is an appropriate technique for the optimization of TE properties. The grains or nanoinclusions of a suitable size can be introduced into the specimen to scatter more phonons than the electrons. In the specimen, the interfaces are rationally designed to reduce their effect on charge mobility (<i>μ)</i> and to decouple it with <i>k_l</i>. While adding hierarchical scattering centers into the heavy-band TE materials, the reduction of maintaining high <i>μ</i> is a key to optimizing <i>ZT</i>. For this, the size of these centers should have a dimension larger than the MFP of the charge carrier but smaller than that of the phonon. Some of the high-performance fabrications described in this review paper exhibiting exceptional TE performance include two-dimensional PbTe quadruple layers, both p-type and n-type materials with enhanced values of 2.39 and 2.44 in the mid-temperature range, respectively, and the hole-doped tin selenide polycrystalline samples with a value of ~ 3.1 at 783 K.