Compliant and stretchable thermoelectric coils for energy harvesting in miniature flexible devices

Abstract

With accelerating trends in miniaturization of semiconductor devices, techniques for energy harvesting become increasingly important, especially in wearable technologies and sensors for the internet of things. Although thermoelectric systems have many attractive attributes in this context, maintaining large temperature differences across the device terminals and achieving low-thermal impedance interfaces to the surrounding environment become increasingly difficult to achieve as the characteristic dimensions decrease. Here, we propose and demonstrate an architectural solution to this problem, where thin-film active materials integrate into compliant, open three-dimensional (3D) forms. This approach not only enables efficient thermal impedance matching but also multiplies the heat flow through the harvester, thereby increasing the efficiencies for power conversion. Interconnected arrays of 3D thermoelectric coils built using microscale ribbons of monocrystalline silicon as the active material demonstrate these concepts. Quantitative measurements and simulations establish the basic operating principles and the key design features. The results suggest a scalable strategy for deploying hard thermoelectric thin-film materials in harvesters that can integrate effectively with soft materials systems, including those of the human body.

Fig 1. 3D thermoelectric coils as active components of flexible and deformable systems for harvesting electrical power.

(A) Schematic illustration of the process for fabrication and 3D assembly. Thin-film p- and n-type materials patterned into 2D serpentine shapes and transferred onto a layer of polyimide (PI) define the active materials. Metal junctions and a top coating of PI patterned by photolithography and etching complete the formation of 2D precursor structures. Chemically bonding such systems to a 60% uniaxially prestretched silicone substrate at selective locations (hot-side junctions), followed by release of the prestretch, initiates a process of geometrical transformation that yields the final 3D architectures. See Materials and Methods and the supplementary materials for details. (B) Optical images of the resulting 3D thermoelectric coils. The geometry of the structure and the elastomer substrate combine to provide mechanical robustness against handling and mechanical deformation. (C) Image of an array with 8 × 8 coils. The magnified view shows that the 3D structure has a geometry consistent with that predicted by FEA. The colored profile represents strain in the silicon leg. The design of the 2D precursor can be found in fig. S2. (D) The 8 × 8 array attached to the skin. Photo credit: Xiwei Shan, UIUC Lab.

Fig 2. Considerations in thermal engineering to optimize choices of design parameters.

(A) The total heat flow across the silicon thermoelectric (TE) leg (blue) increases as the width of the cold side of the encapsulated polymer layer is increased. The overall heat dissipation through surface convection (red) increases the heat flow. (B) Simulated temperature profiles by FEA that compare the encapsulated case (the ratio of the area of the cold to the hot side is 2) to the non-encapsulated case with identical geometry. The encapsulation lowers the temperature at the cold side of the leg. (C) Maximum strain in the thermoelectric leg as a function of the ratio of the area of the cold to the hot side. Increasing the area of the cold side, while desirable for improved performance, compromises the mechanical stability. An area ratio of 2 balances heat exchange capability and mechanical stability. (D) With an area ratio of 2, the leg length is selected to maximize $\mathrm{\Delta }{\mathit{T}}_{\text{TE}}{\stackrel{.}{\mathit{Q}}}_{\text{TE}}$ (right axis, normalized to T_H and area), which is the impedance matching condition. The corresponding fractional temperature drop across the leg is shown on the left axis. All findings presented here are the results of modeling of 3 × 1 coil structures (three leg pairs) with a hot-side thermal bath of 40°C and the entire surface subject to convective heat dissipation due to ambient air at 21°C.

Fig 3. Mechanical deformability and durability of 3D thermoelectric harvesters.

(A) Simulated distributions of strain in the silicon thermoelectric leg before and after uniaxial stretching in the plane by 60%. The results indicate reductions in strain upon stretching, as expected based on the compressive buckling process used to form the 3D structures. (B) Results of experimental durability tests that involve multiple cycles of 60% uniaxial stretch and release on a 3 × 1 coil structure (strain rate ≈ 0.01 s⁻¹). The data indicate only a small increase in the electrical resistance. (C) Optical images (top row) and simulated structures (bottom row) upon in-plane stretching. (D) Simulated values of the maximum local strain in the thermoelectric leg induced by vertical compression. A maximum compression of 26% is possible before reaching the fracture strain of the silicon, the limiting factor for this system. The inset shows the deformed structure upon compression, including a strain distribution map of the silicon leg. The maximum strain occurs near the hot side, indicated as the fracture point (see also fig. S8). (E) Experimental measurements of the device resistance upon vertical compression. The onset of an increase in resistance occurs near the limit predicted by simulation. At a compression of 40%, the device shows open-circuit behavior due to fracture of the silicon.

Fig 4. Energy harvesting with thermoelectric coils and a road map for power enhancement.

(A) Schematic illustration of the measurement conditions for evaluating the performance of the harvesting devices. An 8 × 8 array was exposed to an environmental temperature difference of 19°C. (B) Measured power output characteristics of the 8 × 8 coil array, showing a maximum power of 2 nW. (C) Projected power output achievable by using known thermoelectric materials with thermoelectric figure-of-merit zT higher than that of Si (left axis is for an 8 × 8 array; right axis shows values on a per coil basis). These powers correspond to $\stackrel{.}{\mathit{Q}}\mathrm{\Delta }\mathit{T}/{\mathit{T}}_{\mathrm{H}}$ per leg values as indicated in the legend and ideal conversion efficiencies. The dashed and solid lines represent the values from structures obtainable with organic and inorganic materials, respectively (see fig. S12). The zT values used here correspond to averages of p- and n-type materials reported in the literature: CNT networks (29), PEDOT-Tos (30), TiS₂-organic intercalation (21), Zn₄Sb₃ (44), Mg₃Sb₂ (45), Cu₂Se (46), Ag₂Se (47, 48), and Bi₂Te₃-Sb₂Te₃ (49, 50).