The increasing demand for self-powered wearable electronics has driven research into thermoelectric generators (TEGs) as a promising energy harvesting solution. TEGs convert waste heat into usable electricity, and their efficiency is significantly impacted by minimizing parasitic heat loss and maximizing thermal contact with the heat source. While bismuth telluride (Bi2Te3) alloys are favorable for high-performance TEGs, traditional rigid designs limit their application in wearable devices. Existing flexible TEGs often suffer from poor performance due to heat loss in polymer substrates, poor thermal contact from rigid interconnects, and low manufacturing yield. This research aims to address these limitations by developing compliant TEGs with enhanced heat transfer and unprecedented conformability, paving the way for efficient energy harvesting from arbitrary-shaped heat sources in wearable applications.

Previous research has explored flexible TEGs using various approaches. Infiltration of soft mediums like polydimethylsiloxane (PDMS) into inorganic TEGs offered some mechanical flexibility but resulted in poor performance due to high thermal impedance and air gaps. Liquid metals have been used for soft interconnects, but their instability requires high-thermal-impedance polymer encapsulation, hindering heat transfer. Existing high-performance compliant TEGs often utilize thick and rigid electrodes, limiting flexibility and complicating fabrication. This study addresses these limitations by introducing a novel design incorporating intrinsically stretchable electrodes and soft heat conductors (s-HCs) to improve both performance and conformability.

The researchers developed a soft heat transfer and electrical interconnection platform (SHEP) with intrinsically stretchable electrodes and s-HCs. The s-HCs were created by magnetically self-assembling silver-coated nickel (Ag-Ni) particles in a PDMS matrix, forming vertical chains that significantly improve through-plane thermal conductivity. Silver-nanowire (AgNW)-based stretchable electrodes provide conformability up to 20% strain. A highly automated process, combining simultaneous embedding/patterning/curing of SHEPs with automated epoxy printing and pick-and-place of Bi2Te3 legs, enables the creation of large-area, customizable TEGs. The thermal conductivity of the s-HCs was characterized, and the mechanical properties (Young's modulus, fracture strain) were evaluated. The TE performance of TEGs with and without s-HCs was analyzed using 3D finite element analysis (FEA) and experimental measurements. The mechanical reliability of the TEGs under bending and stretching was also assessed via FEA and experimental testing. Finally, the TEG was integrated into a self-powered wearable warning system, demonstrating its practical application.

The magnetically self-assembled Ag-Ni particle/PDMS composites showed a significant increase in through-plane thermal conductivity (from 0.15 to 1.1 W m⁻¹K⁻¹) compared to non-aligned composites. The s-HCs achieved a through-plane thermal conductivity of ~1.4 W m⁻¹K⁻¹, comparable to that of a Bi2Te3 leg. FEA and experimental results showed that the TEGs with s-HCs exhibited significantly improved performance compared to those without s-HCs, with a 45% increase in open-circuit voltage (Voc) at ΔTApplied of 10 K and a maximum power increase of ~260% at ΔTApplied of 40 K. The automated fabrication process enabled the production of a compliant TEG with 440 TE legs, demonstrating high scalability and customizability. The TEG generated a maximum power of 7.02 mW and a Voc of 2.12 V at a ΔTApplied of 40 K. The compliant TEG exhibited high conformability, forming perfect conformal contact with curved surfaces. FEA simulations showed a ~600% increase in Voc for the compliant TEG compared to a reference TEG with rigid Cu electrodes when attached to a curved heat source. The self-powered wearable system successfully operated LEDs using energy harvested from body heat, demonstrating the practical application of the compliant TEG technology. The compliant TEG also showed excellent mechanical reliability under various bending and stretching conditions with minimal impact on performance.

The results demonstrate that the incorporation of magnetically self-assembled s-HCs and intrinsically stretchable interconnects significantly enhances the performance and conformability of TEGs for wearable applications. The automated manufacturing process offers scalability and customizability, addressing the limitations of previous flexible TEG designs. The achieved high power output and conformability enables the development of practical self-powered wearable devices. The improved heat transfer capability, due to the s-HCs and conformal contact, reduces energy loss and leads to faster response times. The findings highlight the potential of this technology for various self-powered wearable electronics.

This research successfully demonstrated high-performance compliant TEGs with enhanced heat transfer and conformability. The magnetically self-assembled s-HCs and intrinsically stretchable interconnects, along with the automated fabrication process, enabled the creation of highly efficient and customizable TEGs. The self-powered wearable warning system showcased the practical application of this technology. Future work will focus on further reducing the electrical resistance of the stretchable interconnects and contact resistance to improve the overall TEG performance.

The study primarily focused on the performance and reliability of the TEGs under specific bending and stretching conditions. Further investigation is needed to evaluate the long-term stability and durability of the TEGs under various environmental conditions. The optimization of the s-HCs and stretchable electrodes might still improve the overall performance and reduce the internal resistance.