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Mechanically stacked, tandem thermophotovoltaic (TPV) cells featuring integrated air-bridge InGaAs and InGaAsP subcells achieve high spectral efficiency and emission temperature versatility. Thermocompression bonding of electrodes on opposing single air-bridge cells increases out-of-band reflectance (ROUT) compared to cells lacking air bridges. We report a 0.74/0.74 eV homotandem exhibiting ROUT = 96.4%. When operated in a multiterminal arrangement, the homotandem achieves 38% efficiency, marking a 20% absolute improvement over a comparable two-terminal configuration. We also demonstrate a 0.9/0.74 eV heterotandem with ROUT = 97.2% and spectral efficiency approaching 80%. By minimizing losses associated with parasitic absorption and current mismatch, the tandem substantially expands the emission temperature range while preserving high efficiency. This leads to a reduction in the cost of energy storage by over 40%. The air-bridge tandem technology paves the way for high-performance tandem cells compatible with a variety of heat sources unrestricted by the choice of subcell materials. 

Thermophotovoltaic (TPV) cells generate electricity by converting infrared radiation emitted by a hot thermal source. Air-bridge TPVs have demonstrated enhanced power conversion efficiencies by recuperating a large amount of power carried by below-bandgap (out-of-band) photons. Here, we demonstrate single-junction InGaAs(P) air-bridge TPVs that exhibit up to 44% efficiency under 1435°C blackbody illumination. The air-bridge design leads to near-unity reflectance (97-99%) of out-of-band photons for ternary and quaternary TPVs whose bandgaps range from 0.74 to 1.1 eV. These results suggest the applicability of the air-bridge cells to a range of semiconductor systems suitable for electricity generation from thermal sources found in both consumer and industrial applications, including thermal batteries. 

Interest in thermal batteries for inexpensive grid-scale storage of renewable energy motivates the development of photovoltaics that efficiently convert very high temperature thermal emission to electrical energy. We have previously shown that InGaAs air-bridge cells can increase TPV efficiency by −30% compared to cells with more conventional back surface reflectors. In this study, we design and experimentally characterize airbridge cells with wider bandgaps for applications at higher emission temperatures. Parametric studies with varying bandgap and emitter temperature identify high performance regimes. At temperatures up to 2000K, predicted device efficiencies of single-junction air-bridge cells match that of record-holding multi-junction cells. Furthermore, a novel platform for device testing using porous graphite emitters is designed and experimentally demonstrated.