Conjugated polymers have emerged as promising candidates for thermoelectric applications due to their unique electronic properties, low thermal conductivity, and potential for low-cost, large-area processing. Unlike traditional inorganic thermoelectrics, conjugated polymers offer mechanical flexibility, tunable electronic structures, and compatibility with solution-based fabrication methods. Key materials in this field include poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polyacetylene, which have demonstrated significant thermoelectric performance through careful optimization of their electronic and morphological properties.

PEDOT:PSS is one of the most extensively studied conjugated polymers for thermoelectric applications. Its high electrical conductivity, which can exceed 1000 S/cm with appropriate doping, makes it a standout material. The thermoelectric performance of PEDOT:PSS is quantified by the dimensionless figure of merit ZT, defined as ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. PEDOT:PSS achieves ZT values around 0.2 to 0.4 at room temperature, depending on processing conditions and doping levels. The high electrical conductivity arises from the formation of conductive pathways within the PEDOT-rich domains, while the PSS matrix provides solubility and processability. However, the Seebeck coefficient of PEDOT:PSS is relatively low, typically in the range of 10–20 μV/K, which limits ZT. To enhance the thermoelectric performance, researchers have employed strategies such as secondary doping with polar solvents like ethylene glycol or dimethyl sulfoxide, which reorganize the polymer morphology to improve charge transport.

Polyacetylene, the first discovered conductive polymer, also exhibits interesting thermoelectric properties. Although its instability in air has limited its practical applications, doped polyacetylene can achieve high electrical conductivity, exceeding 10,000 S/cm in some cases. The Seebeck coefficient of polyacetylene is higher than that of PEDOT:PSS, often reaching 50–80 μV/K, but its thermal conductivity is also higher, resulting in moderate ZT values. The primary challenge with polyacetylene lies in its environmental instability, as exposure to oxygen and moisture leads to rapid degradation of its electrical properties. Recent efforts have focused on stabilizing polyacetylene through encapsulation or chemical modification, but these approaches often compromise its thermoelectric performance.

To improve the ZT values of conjugated polymers, researchers have explored doping as a primary strategy. Chemical doping introduces charge carriers into the polymer backbone, enhancing electrical conductivity. For PEDOT:PSS, common dopants include sulfuric acid, formic acid, and ionic liquids, which can significantly increase conductivity while moderately affecting the Seebeck coefficient. The trade-off between σ and S is a critical challenge, as increasing carrier concentration typically reduces S due to the Mott relation. Optimizing the doping level is therefore essential to balance these parameters. In polyacetylene, iodine and FeCl₃ are widely used dopants, but their reactivity with environmental factors remains problematic.

Nanostructuring is another effective approach to enhance ZT. By reducing the dimensionality of the polymer or creating nanocomposites, thermal conductivity can be suppressed while maintaining or improving electrical properties. For example, incorporating PEDOT:PSS with carbon nanotubes or graphene can create additional conductive pathways and reduce κ through phonon scattering. Similarly, blending conjugated polymers with insulating polymers or nanoparticles can decouple electronic and thermal transport, leading to higher ZT values. However, achieving uniform dispersion and interfacial compatibility in these nanocomposites remains a technical challenge.

The thermal conductivity of conjugated polymers is inherently low, often below 1 W/mK, which is advantageous for thermoelectric applications. This low κ arises from the disordered nature of polymer chains and weak interchain interactions, which limit phonon propagation. However, the same structural disorder can also hinder charge transport, creating a delicate balance between σ and κ. Strategies such as alignment of polymer chains through mechanical stretching or electric-field-assisted processing have been employed to enhance charge mobility without significantly increasing thermal conductivity.

Stability and scalability are major challenges for conjugated polymer thermoelectrics. Many high-performance polymers, including polyacetylene and heavily doped PEDOT:PSS, are sensitive to moisture, oxygen, and high temperatures, leading to degradation over time. Encapsulation techniques and the development of more stable dopants are critical for real-world applications. Scalability is another concern, as the transition from lab-scale processing to industrial production often introduces variability in material properties. Solution-based techniques like inkjet printing or roll-to-roll coating are promising for large-scale fabrication but require precise control over film morphology and doping uniformity.

Despite these challenges, conjugated polymers offer unique advantages for low-temperature thermoelectric applications, such as wearable energy harvesters or passive cooling devices. Their lightweight, flexible nature enables integration into unconventional form factors, unlike rigid inorganic thermoelectrics. Future research directions include the development of new polymer chemistries with higher intrinsic Seebeck coefficients, improved doping techniques to minimize trade-offs between σ and S, and advanced nanostructuring methods to further reduce κ. Additionally, understanding the long-term stability and degradation mechanisms of these materials under operational conditions will be crucial for commercialization.

In summary, conjugated polymers like PEDOT:PSS and polyacetylene represent a promising class of materials for thermoelectric applications, with ZT values that can compete with some inorganic counterparts in specific temperature ranges. Doping and nanostructuring are key strategies to enhance their performance, but challenges in stability and scalability must be addressed to unlock their full potential. Continued advancements in polymer synthesis, processing, and device engineering will be essential to bridge the gap between laboratory research and practical thermoelectric applications.