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12 Conversion of medium-grade heat (temperature from 500 to 1000 K) into electricity is 13 important in applications such as waste heat recovery, or power generation in solar thermal and 14 co-generation systems. At such temperatures, current solid-state devices lack of either high 15 conversion efficiency (thermoelectrics) or high-power density capacity (thermophotovoltaics 16 and thermionics). Near-field thermophotovoltaics (nTPV) theoretically enables high power 17 density and conversion efficiency by exploiting the enhancement of thermal radiation between a 18 hot emitter and a photovoltaic cell separated by nanometric vacuum gaps. However, significant 19 improvements are possible only at very small gap distances (< 100 nm), and when ohmic losses 20 in the photovoltaic cell are negligible. Both requirements are very challenging for current device 21 designs. In this work, we present a thermionic-enhanced near-field thermophotovoltaic (nTiPV) 22 converter consisting of a thermionic emitter (graphite) and a narrow bandgap photovoltaic cell 23 (InAs) coated with low-workfunction nanodiamond films. Thermionic emission through the 24 vacuum gap electrically interconnects the emitter with the front side of the photovoltaic cell and 25 generates an additional thermionic voltage. This avoids the use of metal grids at the front of the 26 cell, and virtually eliminates the ohmic losses, which are unavoidable in realistic nTPV devices. 27

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