Airbridge cell for ultra-efficient photovoltaic conversion of thermal radiation

State-of-the-art TPV converters use cells with high out-of-band reflectance to facilitate a photon recycle process, in which sub-bandgap photons are reflected by the cell and subsequently re-absorbed at the emitter. However, cells relying on metallic back surface reflectors, Bragg/plasma filters, and photonic crystals for spectral control suffer from undesired out-of-band absorptance and have yet to surpass 95% out-of-band reflectance. Here we describe the fabrication and characterization of a thin-film In0.53Ga0.47As thermophotovoltaic cell with an air-bridge architecture, in which the absorber material is suspended over an air gap, supported by Au grid lines. The average out-of-band reflectance of the cell exceeds 98% due to lossless Fresnel reflectance at the In0.53Ga0.47As-air interface and < 2% loss at the air-Au interface. The result is a record-high TPV conversion efficiency of 32%, characterized under illumination by a 1455K SiC globar.