Conductive polymers have emerged as promising materials for flexible thermoelectric energy harvesting due to their unique combination of electrical conductivity, mechanical flexibility, and solution processability. Among these, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and polyaniline (PANI) stand out as the most extensively studied systems. Their potential in wearable applications stems from their ability to convert body heat into usable electrical energy, enabling self-powered sensors and low-power electronics without relying on rigid inorganic thermoelectrics.
The thermoelectric performance of a material 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. For conductive polymers, achieving high ZT requires optimizing these interdependent parameters. PEDOT:PSS, for instance, exhibits high electrical conductivity (up to 4000 S/cm in heavily doped films) but suffers from a relatively low Seebeck coefficient (typically 10–20 μV/K). Polyaniline, while displaying a higher Seebeck coefficient (20–50 μV/K), often has lower conductivity (1–100 S/cm). Balancing these properties through doping and structural engineering is critical for enhancing thermoelectric efficiency.
Doping strategies play a pivotal role in tuning the thermoelectric properties of conductive polymers. For PEDOT:PSS, secondary doping with polar solvents such as dimethyl sulfoxide (DMSO) or ethylene glycol (EG) significantly enhances conductivity by reorganizing the polymer morphology into a more interconnected network. Post-treatment with acids like sulfuric acid can further improve conductivity by removing insulating PSS chains and increasing crystallinity. The Seebeck coefficient can be independently modulated through redox control. For example, dedoping PEDOT:PSS with hydrazine increases the Seebeck coefficient but reduces conductivity, necessitating a trade-off optimization.
Polyaniline offers additional doping versatility due to its tunable oxidation states (leucoemeraldine, emeraldine, and pernigraniline). Protonic acid doping with camphorsulfonic acid (CSA) or dodecylbenzenesulfonic acid (DBSA) improves both conductivity and processability. The emeraldine salt form of PANI exhibits the highest thermoelectric performance, with ZT values reaching 0.1–0.2 in optimized films. Nanocomposite approaches, such as blending PANI with carbon nanotubes or graphene, have shown promise in decoupling electrical and thermal transport properties to enhance ZT.
Flexibility and mechanical robustness are essential for wearable thermoelectric applications. PEDOT:PSS films retain their conductivity even under repeated bending cycles, with less than 10% degradation after 1000 bends at a 5 mm radius. Polyaniline composites exhibit similar durability when integrated with elastomeric substrates like polydimethylsiloxane (PDMS). The low thermal conductivity of these polymers (0.1–0.5 W/mK) is advantageous for maintaining a temperature gradient across the device, a prerequisite for thermoelectric energy harvesting.
Wearable thermoelectric generators (TEGs) based on conductive polymers typically employ a segmented or lateral architecture to maximize heat flow. A common design involves patterning alternating p-type (PEDOT:PSS or PANI) and n-type (uncommon in polymers but achievable with doped composites) legs on a flexible substrate. For body heat harvesting, a temperature difference of 1–5 K is typical, generating power densities in the range of 0.1–10 μW/cm². While this output is lower than inorganic TEGs, it suffices for low-power wearable electronics such as health monitors or passive sensors.
Recent advances in printing techniques have enabled scalable fabrication of polymer-based TEGs. Screen printing and inkjet printing allow precise deposition of PEDOT:PSS or PANI inks onto textiles or flexible films. Additives like surfactants or cellulose nanofibers improve ink rheology and adhesion. Fully printed TEGs with hundreds of thermocouples have demonstrated open-circuit voltages exceeding 100 mV under realistic temperature gradients.
Challenges remain in improving the environmental stability of conductive polymer TEGs. PEDOT:PSS is hygroscopic, leading to performance degradation in humid conditions. Encapsulation with moisture barriers like Al₂O₃ or parylene can mitigate this issue. Polyaniline suffers from oxidative degradation over time, requiring protective coatings or antioxidant additives. Long-term stability under mechanical stress and repeated thermal cycling also needs further improvement for commercial viability.
Future directions include hybrid systems combining conductive polymers with other organic thermoelectric materials to leverage synergistic effects. For instance, blending PEDOT:PSS with tellurium nanowires has shown enhanced power factors without compromising flexibility. Molecular engineering of new polymer architectures with intrinsically high Seebeck coefficients could bypass the traditional trade-offs between S and σ. Computational screening of dopants and processing conditions may accelerate the discovery of optimized formulations.
The integration of conductive polymer TEGs with energy storage elements, such as supercapacitors or thin-film batteries, is another promising avenue. This would enable continuous operation of wearable devices by storing intermittent thermoelectric energy. Smart textiles with woven thermoelectric fibers could harvest energy from both body heat and ambient temperature fluctuations, powering embedded sensors without external batteries.
In summary, conductive polymers like PEDOT:PSS and polyaniline offer a compelling pathway toward flexible, wearable thermoelectric energy harvesting. Through strategic doping, nanocomposite engineering, and advanced fabrication techniques, their performance continues to approach practical utility. While challenges in stability and efficiency persist, ongoing research holds the potential to unlock fully autonomous, self-powered wearable systems powered by body heat.