Thermoelectric materials have emerged as a promising solution for energy harvesting, converting waste heat directly into electricity through the Seebeck effect. This phenomenon occurs when a temperature gradient across a material induces a voltage difference, enabling the generation of electrical power without moving parts. The efficiency of thermoelectric 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. High-performance thermoelectric materials require a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity, a combination that is challenging to achieve due to the interdependence of these parameters.

Bismuth telluride (Bi₂Te₃) is one of the most widely studied thermoelectric materials, particularly for near-room-temperature applications. Its layered crystal structure contributes to low lattice thermal conductivity, while its tunable electronic properties allow optimization of the Seebeck coefficient and electrical conductivity. Alloying Bi₂Te₃ with antimony (Sb) and selenium (Se) further enhances its ZT by reducing thermal conductivity through point defect scattering. For example, p-type Bi₀.₅Sb₁.₅Te₃ exhibits a ZT of approximately 1.8 at 320 K, making it suitable for cooling applications and low-grade waste heat recovery. N-type Bi₂Te₂.₇Se₀.₃ achieves a ZT of around 1.0 in the same temperature range, balancing electrical and thermal transport properties.

Skutterudites, another class of thermoelectric materials, are particularly effective at intermediate temperatures (500–900 K). These materials, with the general formula MX₃ (where M is a transition metal like Co or Fe, and X is a pnictogen like Sb or As), feature a cage-like crystal structure that can be filled with rare-earth or alkaline-earth atoms. These filler atoms rattle within the cages, scattering phonons and significantly reducing lattice thermal conductivity without severely degrading electrical conductivity. For instance, Yb-filled CoSb₃ achieves a ZT of 1.5 at 800 K, making it suitable for automotive waste heat recovery systems. The ability to tailor filler content and composition allows precise control over thermoelectric performance.

Organic thermoelectric materials offer advantages in flexibility, lightweight design, and low-cost processing, though their ZT values are generally lower than inorganic counterparts. Conjugated polymers such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) have shown promise due to their tunable electrical conductivity and low thermal conductivity. By optimizing doping levels and polymer chain alignment, researchers have achieved ZT values approaching 0.4 at room temperature. While this is lower than Bi₂Te₃ or skutterudites, organic thermoelectrics are well-suited for applications requiring mechanical flexibility, such as wearable energy harvesters.

Enhancing ZT values remains a central challenge in thermoelectric research. One effective strategy is nanostructuring, which reduces thermal conductivity by increasing phonon scattering at grain boundaries or interfaces. For example, incorporating nanoscale precipitates in PbTe-based materials has led to ZT values exceeding 2.0 at 900 K. Similarly, superlattices of Bi₂Te₃/Sb₂Te₃ exploit interfacial scattering to suppress thermal conductivity while maintaining electrical performance. Another approach involves band engineering to increase the Seebeck coefficient. Resonant doping, where impurity levels align with the conduction or valence band edges, can enhance the density of states near the Fermi level, improving the power factor (S²σ).

Doping plays a critical role in optimizing thermoelectric properties. In inorganic materials, controlled doping adjusts carrier concentration to maximize the power factor. For instance, n-type Bi₂Te₃ benefits from halogen doping (e.g., I or Br), which provides additional electrons and improves electrical conductivity. In organic thermoelectrics, molecular doping with strong oxidants or reductants can significantly enhance conductivity. However, excessive doping can reduce the Seebeck coefficient due to increased carrier scattering, necessitating a careful balance.

Applications of thermoelectric materials span waste heat recovery and micro-power generation. Industrial processes, automotive exhaust systems, and power plants generate vast amounts of waste heat, much of which is lost to the environment. Thermoelectric generators (TEGs) can convert a portion of this heat into usable electricity, improving overall energy efficiency. For example, TEGs integrated into vehicle exhaust systems can recover enough energy to power auxiliary electronics, reducing fuel consumption. In micro-power generation, thermoelectric modules power wireless sensors and IoT devices by harvesting heat from ambient sources like machinery or even the human body.

The development of high-ZT materials continues to advance through interdisciplinary research. Novel material systems, such as complex chalcogenides and hybrid organic-inorganic composites, are being explored to overcome the limitations of traditional thermoelectrics. Machine learning and high-throughput computational screening are accelerating the discovery of new compositions with optimized properties. As thermoelectric technology matures, its integration into energy systems will play a crucial role in sustainable energy management, reducing reliance on fossil fuels and mitigating environmental impact.

In summary, thermoelectric materials leverage the Seebeck effect to harvest energy from temperature gradients, with performance dictated by ZT. Bi₂Te₃, skutterudites, and organic thermoelectrics each serve distinct temperature ranges and applications. Advances in nanostructuring, doping, and band engineering continue to push the boundaries of ZT, enabling more efficient waste heat recovery and micro-power generation. The ongoing development of these materials holds significant potential for sustainable energy solutions across industries.