Air-Bridge Si Thermophotovoltaic Cell with High Photon Utilization

Thermophotovoltaic (TPV) cells convert photons emitted from hot surfaces into electrical power. Unlike solar cells, TPV cells can be placed in close proximity to the heat source, allowing below-bandgap (i.e., out-of-band, OOB) photons to be reflected and reabsorbed by the emitter. As the reflectance of OOB photons approaches unity, the spectral efficiency of the TPV becomes increasingly insensitive to the bandgap of the cell and the source temperature. Here, we employ air-bridge structures with a lateral junction Si TPV cell as a means of increasing OOB reflectivity, which allows for efficient operation at low thermal source temperatures that were long inaccessible to efficient Si TPV power generation. The devices exhibit an OOB reflectance of 98.0 ± 0.1% and an air-bridge scalability to at least 6 cm × 6 cm. Compared to devices with a Au back surface reflector (BSR), devices featuring an air-bridge BSR exhibit a 25% relative increase in power conversion efficiency at a thermal source temperature of 1988 K. Such performance improvements in TPV cells made with scalable and relatively low-cost Si can potentially expedite the widespread use of TPV systems in both energy storage and generation systems.