Thermal-to-electrical energy conversion by diodes under negative illumination

We consider an infrared photodiode under negative illumination, wherein the photodiode is maintained at a temperature $T$ and radiatively exposed to an emissive body colder than itself. We experimentally demonstrate that a diode under such conditions can generate electrical power. We show theoretically that the efficiency of energy conversion can approach the Carnot limit. This work is applicable to waste heat recovery as well as emerging efforts to utilize the cold dark universe as a thermodynamic resource for renewable energy.