Demonstration of thermal energy conversion though radiating optical rectennas for night harvesting applications

Night harvesting, where heat is harvested from the temperature difference between Earth and deep space, promises clean energy when sunlight is not available. The low efficiency of available thermal-to-electrical energy conversion technologies has been the largest barrier in the commercial pursuit of this technology. We present a new approach using radiating optical rectennas – femtosecond-fast metal-insulator-metal (MIM) diodes and micron-scale antennas – to overcome the limitations of low efficiency for low temperature differences (< 100 °C). We experimentally demonstrate the basic principle of thermal-to-electrical energy conversion using a Ni-NiO-Al₂O₃-Cr-Au MIM diode-based optical rectenna, with a transmission line compensation structure to enhance the coupling efficiency between antenna and diode at terahertz frequencies. For a temperature gradient of 80° C, the device generates an open circuit voltage of 300 mV, corresponding to a power of at least 93 mW/m². Under practical operating conditions, this technology has the potential to exceed similar technologies exploiting radiative cooling.