Nano Lett. 18, 5224–5230 (2018)

Thermal electromagnetic radiation can be converted with relatively high efficiency thanks to photovoltaic technology, but the power density of thermal radiation is low compared with what is possible with the conduction-based generators. Bo Zhao and colleagues from the USA tackle this problem by proposing a device that exploits evanescent fields that may offer simultaneously high power density (9.6 W cm–2) and thermal-to-electrical conversion efficiency (9.8%). Their solid-state thermophotonic system uses an AlGaAs light-emitting diode (LED) (hot side) and an AlGaAs photovoltaic (PV) cell (cold side), both 900 nm thick, with Ag on the back-sides of each acting as reflectors, critical for the achieved efficiency. Emission from the LED is captured by the PV cell and some of the generated power biases and drives the LED. The trick to achieving high power density is working in a near-field regime with a gap spacing of only 10 nm between the 600 K hot side and the 300 K cold side, as well as appropriate layer thicknesses to reduce non-radiative recombination. This temperature range represents what is known as ‘low-grade’ heat waste where thermoelectric devices dominate, but the proposed thermophotonic device may outperform these.