Layer Exchange Synthesis of SiGe for Flexible Thermoelectric Generators: A Comprehensive Review

Abstract

Flexible thermoelectric generators are leading candidates for next‐generation energy‐harvesting devices. Although SiGe, an environmentally‐friendly semiconductor, is the most reliable and widely tested thermoelectric material, it is difficult to form a SiGe layer with high thermoelectric performance at temperatures lower than the heat‐proof temperature of flexible plastic films. In this article, the synthesis of SiGe thermoelectric thin films via the metal‐induced layer exchange phenomenon is reviewed, from its mechanism to device performance. The selection of metal species allows low‐temperature formation (≤500 °C) of p‐ and n‐type SiGe on insulating substrates. Currently, the maximum power factors near room temperature are 850 µW m−1 K−2 for p‐type Si0.4Ge0.6 and 1000 µW m−1 K−2 for n‐type Si0.85Ge0.15. These values are the highest among those of Group IV semiconductor thin films formed at low temperatures. The flexible thermoelectric generator consisting of these p‐ and n‐type SiGe exhibits cross‐sectional and planar power densities of ≈3.0 mW cm−2 and 0.50 µW cm−2, respectively, at a temperature difference of 30 K. Finally, the future challenges of layer exchange for improving the performance of flexible thermoelectric generators based on Group IV semiconductors are discussed.