Nature Photonics nature photonics https://doi.org/10.1038/s41566-024-01537-5 Review article Semiconductor thermoradiative power conversion Michael P. Nielsen  1,3 , Andreas Pusch  1,3 , Phoebe M. Pearce  1 , Muhammad H. Sazzad  1 , Peter J. Reece  2 , Martin A. Green  1 & Nicholas J. Ekins-Daukes  1 Power can be generated from radiative exchange between two bodies with different temperatures—from the radiative cooling of the Earth’s surface into space, for example. Thermoradiative diodes are low-bandgap optoelectronic devices in which the occupancies of the valence and conduction bands are established through radiative exchange with the external environment. A warm diode viewing cold surroundings will spontaneously develop a reverse electrical bias, which, combined with the recombination current from the radiative imbalance, generates electrical power. Here we review the operating principles of the thermoradiative diode in both the radiative limit and in the presence of non-radiative processes. We discuss some present limitations and opportunities for improved performance together with potential applications such as night-sky power generation and waste-heat recovery. The capacity for thermoradiative power generation arises from the emission of light from a warm body into a colder environment. Ther- modynamically, work can always be performed between any radiatively coupled difference in temperatures, so power can be generated either by absorption of a hot radiative flux as in conventional solar power or emission into a cold radiative ambient, known as thermoradiative power. This concept of thermoradiative power generation has been used for decades in the radioisotope thermoelectric generators used to power deep-space probes 1 . As illustrated in Fig. 1a, heat at a temperature T H flows through a thermoelectric generator before being radiated into cold space from an emissive surface at temperature T EM , which is colder than the heat source T H but warmer than space at T C . The concept has also recently been used to generate electrical power from the radiation released by the warm Earth into outer space 2–4 . However, if a low-bandgap semiconductor is used as the thermal emitter, it becomes possible to generate electrical power directly, without an intermediate temperature, as illustrated in Fig. 1b 5,6 . If the semiconductor is maintained at temperature T H then the presence of the cold environment establishes a quasi-thermal carrier population with a carrier density below that at equilibrium in the conduction and valence bands described by a Fermi energy splitting Δμ and hence the opportunity to deliver electrical power 7,8 . The situation is analogous to that of a semiconductor photovoltaic solar cell whereby the high-temperature radiant heat from the Sun is converted directly into electricity by establishing a quasi-thermal carrier population above that at equilibrium in the semiconductor bands. The difference between the solar photovoltaic device and the thermoradiative device is that, for the photovoltaic solar cell exposed to a hot radiative environ- ment (the Sun), there is an excess carrier population in the bands and a positive Fermi energy splitting, whereas for a thermoradiative device exposed to a cold radiative environment there is a carrier population deficit in the bands and hence a negative Fermi energy splitting, as identified by Shockley and Queisser in their seminal work 9 . In both cases, the availability of free energy ̇ W in the form of electrical power arises as, for each unit of energy ̇ Q, the net entropy flux associated with radiative emission (S out − S in ) is higher than that associated with the heat flow from the reservoir to the emitter ̇ Q H /T H (ref. 10). In 2016, a photocurrent supplied by a thermoradiative diode was reported for a mercury cadmium telluride (HgCdTe) diode operat- ing at room temperature and exposed to a colder radiative environ- ment 11 . In 2022, the full current–voltage curves from a series of similar HgCdTe diodes with different bandgaps were reported 12 . Considering the relatively recent emergence of the thermoradiative diode, with a limited number of experimental demonstrations and theoretical Received: 26 November 2023 Accepted: 15 August 2024 Published online: xx xx xxxx Check for updates 1 School of Photovoltaic & Renewable Engineering, UNSW Sydney, Kensington, New South Wales, Australia. 2 School of Physics, UNSW Sydney, Kensington, New South Wales, Australia. 3 These authors contributed equally: Michael P. Nielsen, Andreas Pusch. e-mail: nekins@unsw.edu.au