Highly efficient transverse thermoelectric devices with Re4Si7 crystals

Abstract

The principal challenges in current thermoelectric power generation modules are the availability of stable, diffusion-resistant, lossless electrical and thermal metal–semiconductor contacts that do not degrade at the hot end nor cause reductions in device efficiency. Transverse thermoelectric devices, in which a thermal gradient in a single material induces a perpendicular voltage, promise to overcome these problems. However, the measured material transverse thermoelectric efficiencies, z_xyT, of nearly all materials to date has been far too low to confirm these advantages in an actual device. Here, we show that single crystals of Re₄Si₇, an air-stable, thermally robust, layered compound, have a transverse z_xyT of 0.7 ± 0.15 at 980 K, a value that is on par with existing commercial longitudinal theremoelectrics today. Through constructing and characterizing a transverse power generation module, we prove that extrinsic losses through contact resistances are minimized in this geometry, and that no electrical contacts are needed at the hot side. This excellent transverse thermoelectric performance arises from the large, oppositely signed in-plane p-type and cross-plane n-type thermopowers. These large anisotropic thermopowers arise from thermal population of the highly anisotropic valence band and isotropic conduction band in this narrow gap semiconductor. Overall, this work establishes Re₄Si₇ as the “gold-standard” of transverse thermoelectrics, allowing future exploration of unique device architectures for waste heat recovery.