Thermionic-enhanced near-field thermophotovoltaics for medium-grade heat sources

Conversion of medium-grade heat (temperature from 500 to 1000 K) into electricity is important in applications such as waste heat recovery or power generation in solar thermal and co-generation systems. At such temperatures, current solid-state devices lack either high conversion efficiency (thermoelectrics) or high-power density capacity (thermophotovoltaics and thermionics). Near-field thermophotovoltaics (nTPV) theoretically enables high-power density and conversion efficiency by exploiting the enhancement of thermal radiation between a hot emitter and a photovoltaic cell separated by nanometric vacuum gaps. However, significant improvements are possible only at very small gap distances (<100 nm) and when ohmic losses in the photovoltaic cell are negligible. Both requirements are very challenging for current device designs. In this work, we present a thermionic-enhanced near-field thermophotovoltaic (nTiPV) converter consisting of a thermionic emitter (graphite) and a narrow bandgap photovoltaic cell (InAs) coated with low-workfunction nanodiamond films. Thermionic emission through the vacuum gap electrically interconnects the emitter with the front side of the photovoltaic cell and generates an additional thermionic voltage. This avoids the use of metal grids at the front of the cell and virtually eliminates the ohmic losses, which are unavoidable in realistic nTPV devices. We show that nTiPV operating at 1000 K and with a realizable vacuum gap distance of 100 nm enables a 10.7-fold enhancement of electrical power (6.73 W/cm²) and a 2.8-fold enhancement of conversion efficiency (18%) in comparison with a realistic nTPV device having a series resistance of 10 mΩ·cm².