The Effect of Area Density of Polysilicon Thermocouples on Thermoelectric Performance

TL;DR

This paper shows how increasing the area density of polysilicon thermocouples improves the performance of thermoelectric generators for wearable devices.

Contribution

The study introduces area density as a better performance metric than fill factor for thermoelectric generators.

Findings

A TEG chip with 57 × 1 μm stacked thermocouples achieves an area density of 8621 thermocouples per mm².

A 5 × 5 mm² TEG chip can generate over 3 μW and 3 V at a 10 °C temperature difference.

Stacked thermocouples improve both power factor and voltage factor compared to co-planar designs.

Abstract

Thermoelectric energy generators (TEGs) that can convert body heat into electricity are considered most promising to drive wearable devices. Many TEG designs with a polysilicon thermocouple have been proposed for implementation in high-yield semi-conductor foundry services. This study shows that the area density, defined by the number of thermocouples per mm2, is a better index than the fill factor in evaluating TEG performance. The effects of thermocouple length, width, and spacing (between the adjacent thermocouples) on area density, and hence on TEG performance, are analyzed. For a TEG with 33 × 1 μm (length × width) co-planar thermocouples (P- and N-thermoleg side by side) and 1 μm spacing between two adjacent thermocouples, the area density is 4902 thermocouples per mm2 and it can deliver a 0.110 μW/cm2K2 power factor and a 12.906 V/cm2K voltage factor. The performance can be improved further by 57 × 1 μm stacked thermocouples (P-thermoleg above N-thermoleg) with a higher area density 8621 to achieve results of 0.110 μW/cm2K2 and 22.638 V/cm2K. Such a high area density not only increases TEG performance, but also improves the DC–DC converter efficiency. A 5 × 5 mm2 TEG chip with co-planar or stacked thermocouples is shown to deliver above 3 μW and over 3 V when operating at a 10 °C temperature difference.

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