Thermoradiative devices enabled by hyperbolic phonon polaritons at nanoscales

Theromoradiative (TR) devices, though proposed a decade ago, have seen little investigation due to their lowperformance compared to other solid-state energy conversion technologies. Herein, we propose an InSb-hBN TR device with a nanoscale vacuum gap down to 10 nm that has the potential to achieve efficient waste heat recovery using solid-state technologies. By coupling the hyperbolic phonon polaritons of hBN with the interband transition of InSb, this TR device design can achieve an output power nearly 4 orders-of-magnitude higher than far-field scenarios and an upper bound efficiency up to 80% of the Carnot limit corresponding to only considering radiative loss. A detailed balance model combined with fluctuational electrodynamics for a multilayered anisotropic structure is developed to theoretically estimate the performance of TR devices from far- to near-field regime. Nonradiative losses are also investigated to elucidate their effects on performance. This work helps deepen the understanding of TR devices and provides a promising pathway to efficient low-grade (< 600 K) heat recovery.