(251g) Air-Bridge Architectures for Record-High Efficiency In0.53Ga0.47As Thermophotovoltaic Cells

Solid-state thermophotovoltaic (TPV) converters are promising for scalable, on-demand power generation. With improved conversion of thermal radiation into electricity, this technology may enable implementation of less wasteful energy generation strategies (e.g., distributed co-generation) and more affordable energy storage techniques (e.g., grid-scale thermal energy storage) that are not currently supported by conventional, mechanical heat engines. Absorption of above-bandgap (in-band) radiation in the cell is necessary for achieving the technology’s characteristically high power densities. To achieve high conversion efficiencies, however, suppression of sub-bandgap (out-of-band) radiation is critical for mitigating parasitic loss mechanisms below the bandgap, where most radiated power exists. State-of-the-art TPV converters utilize 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. Reflective cells enable superior spectral utilization compared to competing approaches, such as selective emitters, which suffer from thermal instabilities. 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 report the fabrication and characterization of a thin-film In_0.53Ga_0.47As 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 In_0.53Ga_0.47As-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. We further observe that spectral utilization becomes less sensitive to cell bandgap and emitter temperature in this high reflectance regime. This may enable use of more affordable, wider bandgap PV materials, like Si, for TPV converters at moderate emitter temperatures.