Chalcogenide semiconductors have emerged as promising candidates for thermoelectric applications due to their tunable electronic properties and intrinsically low lattice thermal conductivity. These materials, primarily composed of sulfur, selenium, or tellurium combined with elements like bismuth, antimony, or lead, exhibit favorable charge transport characteristics that can be further optimized for energy conversion efficiency. The thermoelectric performance of a material is quantified by the dimensionless figure of merit, ZT, defined as (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity. Achieving high ZT requires balancing these interdependent parameters, often through strategic doping and nanostructuring.

Doping plays a critical role in optimizing the electrical transport properties of chalcogenides. For instance, p-type doping in Bi₂Te₃ with antimony enhances the power factor (S²σ) by modifying the density of states near the Fermi level. Similarly, n-type doping with iodine or copper in PbTe has been shown to increase carrier concentration without significantly degrading mobility. The selection of dopants must consider both the electronic structure of the host material and the defect chemistry introduced by the dopant. In some cases, resonant doping—where dopant energy levels align closely with the host's band edges—can dramatically improve the Seebeck coefficient. For example, thallium-doped PbTe achieves ZT values exceeding 1.5 at 700 K due to resonance levels that enhance the density of states effective mass.

Nanostructuring is equally important for reducing lattice thermal conductivity (κₗ) without adversely affecting electronic transport. Chalcogenides inherently exhibit low κₗ due to their complex crystal structures and heavy constituent atoms, but further reduction can be achieved through grain boundary engineering. Introducing nanoscale precipitates or creating superlattices effectively scatters mid- to long-wavelength phonons while minimizing electron scattering. In BiSbTe alloys, nanostructuring has reduced κₗ to values as low as 0.4 W/m·K, contributing to ZT values above 1.4 at room temperature. Another approach involves designing hierarchical architectures where phonons are scattered across multiple length scales, from atomic-scale point defects to mesoscale grain boundaries.

Alloying is another strategy to suppress κₗ by introducing mass contrast and strain field fluctuations. For example, in PbTe₁₋ₓSeₓ solid solutions, the random distribution of selenium and tellurium atoms creates significant phonon scattering, reducing κₗ by over 50% compared to pure PbTe. The alloying process must be carefully controlled to avoid phase separation, which can degrade electronic properties. Quaternary systems like AgSbTe₂-Se₂ have demonstrated ZT enhancement due to simultaneous optimization of electronic and thermal transport through entropy engineering.

Device integration of chalcogenide thermoelectrics presents several challenges, primarily related to thermal stability and contact resistance. Many high-performance chalcogenides, such as SnSe, exhibit anisotropic thermal and electrical properties, requiring careful alignment of crystallographic directions during module assembly. The large thermal expansion mismatch between chalcogenides and metal electrodes can lead to mechanical failure during thermal cycling. Diffusion barriers are often necessary to prevent interfacial reactions; for instance, nickel layers are used to block copper diffusion in Bi₂Te₃-based devices.

Another integration challenge lies in maintaining performance stability at elevated temperatures. Some chalcogenides undergo phase transitions or decomposition when operated near their melting points. For example, GeTe-rich alloys experience a rhombohedral-to-cubic phase transition around 700 K, which alters their transport properties. Compositional engineering with additives like manganese can stabilize the desired phase over wider temperature ranges. Oxidation is another concern for tellurium-containing compounds, necessitating protective coatings or hermetic packaging in practical applications.

The development of reliable n-type and p-type pairs with matched thermal expansion coefficients and compatible interfacial properties remains an ongoing research focus. While Bi₂Te₃-based materials show excellent performance near room temperature, their ZT declines sharply above 500 K. In contrast, materials like PbTe and SnSe maintain high performance at mid-to-high temperatures but require different doping strategies for n-type and p-type legs. Recent advances in segmented modules, where different materials are used along the temperature gradient, have improved conversion efficiency but add complexity to fabrication and reliability testing.

Future directions in chalcogenide thermoelectrics research include exploring new material systems with intrinsically low κₗ, such as complex sulfides with liquid-like phonon transport. The discovery of materials with anharmonic bonding or rattling atoms, like in Cu₂Se, has opened new avenues for thermal conductivity reduction without electronic property degradation. Machine learning approaches are being employed to identify optimal doping combinations and nanostructuring parameters that simultaneously enhance power factor and reduce κₗ. Another promising area is the development of flexible thermoelectric devices using chalcogenide thin films, which could enable waste heat recovery from curved surfaces or wearable applications.

Environmental considerations are becoming increasingly important in chalcogenide thermoelectric development. While tellurium is a rare element, research into earth-abundant alternatives like tin sulfide (SnS) has shown progress, with ZT values approaching 1.0 in optimized samples. Sulfur-based chalcogenides offer advantages in terms of material availability and reduced toxicity compared to tellurium or lead-containing compounds. However, these systems often require more aggressive doping and nanostructuring to compensate for their typically higher κₗ values.

The scalability of high-ZT chalcogenide materials remains a hurdle for widespread commercialization. Many laboratory-scale synthesis methods, such as spark plasma sintering or molecular beam epitaxy, are difficult to translate to industrial production. Solution-based processing techniques are being investigated as potential low-cost alternatives, though they often struggle to achieve the same level of structural control as vacuum-based methods. Quality control during large-scale synthesis, particularly in maintaining consistent nanostructure distributions, is another challenge that requires attention.

In summary, chalcogenide semiconductors continue to demonstrate remarkable potential for thermoelectric energy conversion through careful optimization of their electronic and thermal transport properties. The interplay between doping strategies and nanostructure engineering has enabled significant improvements in ZT values across various temperature ranges. While challenges remain in device integration and manufacturing scalability, ongoing research into new material systems and processing techniques promises to further advance the field. The unique properties of chalcogenides position them as key materials for sustainable energy harvesting applications, provided that stability and cost considerations can be adequately addressed.