Enhanced Photon Utilization in Single Cavity Mode Air-Bridge Thermophotovoltaic Cells

An air-bridge thermophotovoltaic(TPV) cell can enablenearly completeutilization of out-of-band (OOB) photons. However, the air-bridgeconsists of a micron-thick free-standing semiconductor membrane thatcan buckle during fabrication. Such a buckled membrane supports multipleoptical cavity modes in the air gap between the semiconductor andthe bottom, metal back surface reflector, causing up to 10% loss inOOB reflectance (R (OOB)). Here we demonstratea single cavity mode with an extremely flat In0.53Ga0.47As TPV membrane that exhibits R (OOB) = 98.9 +/- 0.1% under 1279 K blackbody illumination. The remaining1.1% reflectance loss is attributed to free carrier absorption andcavity oscillations. The flat TPV cell exhibits a spectral efficiencyranging from 68.6% to 76.5% for emitter temperatures between 900 and1500 K, which exceeds that of previous reflective TPV cells, and representsa 20-30% improvement compared to the buckled cell, leadingto a power conversion efficiency of 31.7 +/- 0.1% at 1279 K.