Limits to the Energy-Conversion Efficiency of Air-Bridge Thermophotovoltaics

Air-bridge thermophotovoltaic (ABTPV) devices demonstrate near-99% utilization of photons whose energy is less than the semiconductor band gap (known as out-of-band, OOB, photons). Here, we determine 55.5% is the thermodynamic limit at an emitter temperature of 1400 K for single junction ABTPVs based on detailed balance, which compares to a thus-far reported efficiency of 32%. A technology computer-aided design model for the (In,Ga)As diode predicts that suppressing the surface-recombination velocity to <200 cm/s significantly improves the open-circuit voltage and the shunt resistance, resulting in 48.6% as the practical efficiency limit. We find that a 1% OOB loss due to free-carrier absorption in doped regions of the TPV can drastically decrease the efficiency at larger band gaps and lower emitter temperature. We also analyze the effects of shadowing by metal grids, which generally introduces only minor losses in TPV performance.