Thermoelectric materials convert heat into electricity and vice versa, offering potential applications in waste heat recovery, solid-state cooling, and power generation. The efficiency of these materials is quantified by the dimensionless figure of merit, ZT, defined as ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity. Enhancing ZT requires optimizing these interdependent parameters, which presents a significant challenge due to their conflicting nature. Recent advances in band engineering, doping, nanostructuring, and phonon scattering have led to notable improvements in ZT values. This article explores these strategies and their role in designing high-performance thermoelectric materials.

Band engineering plays a crucial role in enhancing the Seebeck coefficient and electrical conductivity. The Seebeck coefficient is sensitive to the density of states near the Fermi level, and manipulating the band structure can lead to higher power factors (S²σ). One approach involves creating resonant states or band convergence, where multiple bands contribute to carrier transport. For instance, introducing impurity levels near the Fermi level can increase the density of states, improving the Seebeck coefficient without severely degrading electrical conductivity. Another strategy is to engineer band degeneracy, where multiple valleys in the conduction or valence bands contribute to carrier transport, enhancing the power factor. These methods require precise control over the electronic structure, often achieved through alloying or strain engineering.

Doping is another critical factor in optimizing thermoelectric performance. By carefully selecting dopants, the carrier concentration can be tuned to maximize the power factor while minimizing detrimental effects on thermal conductivity. Heavy doping can improve electrical conductivity but may also increase electronic thermal conductivity, offsetting gains in ZT. Optimal doping levels depend on the material system and operating temperature range. For example, in p-type materials, acceptor dopants are used to increase hole concentration, while n-type materials rely on donor dopants. The choice of dopant also affects defect scattering, which can influence carrier mobility. Recent studies have explored co-doping strategies, where multiple dopants are used to simultaneously optimize carrier concentration and mobility.

Nanostructuring has emerged as a powerful tool to reduce thermal conductivity without significantly impairing electrical properties. Thermal conductivity consists of lattice (κ_l) and electronic (κ_e) contributions. Since κ_e is linked to electrical conductivity, efforts focus on minimizing κ_l through phonon scattering. Nanostructured materials, such as superlattices and nanocomposites, introduce interfaces and grain boundaries that scatter phonons more effectively than electrons. In superlattices, alternating layers of different materials create periodic interfaces that disrupt phonon propagation while maintaining coherent electronic transport. Nanocomposites, on the other hand, incorporate nanoscale inclusions or secondary phases that scatter phonons across a wide range of frequencies. These approaches have led to significant reductions in lattice thermal conductivity, pushing ZT values beyond traditional limits.

Phonon scattering mechanisms are central to reducing thermal conductivity. Point defects, dislocations, and grain boundaries act as scattering centers for phonons, with their effectiveness depending on the phonon wavelength. High-frequency phonons are scattered by atomic-scale defects, while mid- and low-frequency phonons are affected by larger-scale structures like grain boundaries. Hierarchical structuring, which incorporates scattering centers across multiple length scales, has proven particularly effective. For example, materials with embedded nanoparticles and micro-scale grain boundaries can scatter a broad spectrum of phonons, leading to ultralow thermal conductivity. Recent advances in defect engineering have enabled precise control over phonon scattering, further enhancing ZT.

Bulk thermoelectric materials remain widely used due to their scalability and mechanical robustness. However, their performance is often limited by high thermal conductivity. To address this, researchers have developed bulk materials with intrinsic low thermal conductivity, such as those with complex crystal structures or strong anharmonicity. Alloying is a common strategy to introduce disorder and reduce κ_l. For instance, solid solutions with mass contrast between constituent atoms enhance phonon scattering. Additionally, designing materials with rattling atoms or lone pair electrons can suppress heat transport by increasing anharmonic vibrations. These approaches have yielded bulk materials with ZT values competitive with nanostructured systems.

Superlattices offer unique advantages for thermoelectric applications by combining quantum confinement effects with interface engineering. In these structures, alternating thin layers of different materials create periodic potential barriers that confine carriers and modify their transport properties. Quantum wells formed in superlattices can enhance the density of states near the Fermi level, improving the Seebeck coefficient. Meanwhile, the interfaces between layers scatter phonons effectively, reducing thermal conductivity. Recent work has demonstrated that carefully designed superlattices can achieve ZT values exceeding those of bulk materials, particularly in thin-film configurations. However, challenges remain in scaling up production and maintaining stability at high temperatures.

Nanocomposites integrate nanoscale features into a bulk matrix, combining the benefits of nanostructuring with practical manufacturability. These materials often consist of a host matrix with embedded nanoparticles or nanowires, which introduce additional phonon scattering interfaces. The distribution and size of these inclusions are critical; uniform dispersion ensures consistent phonon scattering, while optimized sizes target specific phonon wavelengths. Nanocomposites also benefit from energy filtering effects, where low-energy carriers are scattered more strongly than high-energy ones, enhancing the Seebeck coefficient. Advances in synthesis techniques have enabled precise control over nanocomposite morphology, leading to record-high ZT values in some systems.

Despite these advances, challenges persist in achieving high ZT values across a wide temperature range. Many high-ZT materials exhibit peak performance only at specific temperatures, limiting their practical utility. Thermal stability is another concern, as nanostructured materials may degrade at elevated temperatures due to grain growth or interdiffusion. Mechanical properties also play a role, particularly in applications requiring durability under thermal cycling. Future research directions include exploring new material systems with intrinsically low thermal conductivity, developing advanced characterization techniques to probe phonon and carrier dynamics, and integrating machine learning for accelerated material discovery.

Recent breakthroughs have pushed ZT values above 2 in select materials, a significant milestone for commercial applications. However, translating laboratory achievements into scalable technologies requires addressing cost, reproducibility, and environmental impact. The interplay between material properties and device engineering also demands attention, as optimal ZT does not always translate to optimal device efficiency. Continued collaboration between theorists, experimentalists, and engineers will be essential to overcome these hurdles and unlock the full potential of thermoelectric materials.

In summary, the design and optimization of thermoelectric materials rely on a multifaceted approach that balances electronic and thermal transport properties. Band engineering, doping, nanostructuring, and phonon scattering each contribute to enhancing ZT, with recent advances demonstrating the viability of surpassing traditional performance limits. While challenges remain in scalability and stability, ongoing research holds promise for realizing efficient and practical thermoelectric technologies.