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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) systems are a promising technology for power generation from thermal sources. Understanding how the efficiency of emerging TPVs is reported and how it relates to expected performance under realistic operating conditions is important for deployment of this technology and can inform future progress in the field. This work provides an overview of the main efficiency characterization techniques used today, including full and partial TPV irradiance, and direct and indirect (optical) heat transfer measurements. The overall steps of each method are discussed, along with their respective advantages and shortcomings from the perspective of mimicking practical operation. By surveying current methodologies, this study highlights areas for improvement and seeks to guide in selecting and refining efficiency characterization methods for TPV systems. 

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. 

We present a radiative cooling material capable of enhancing albedo while reducing ground surface temperatures beneath fielded bifacial solar panels. Electrospinning a layer of polyacrylonitrile nanofibers, or nanoPAN, onto a polymer-coated silver mirror yields a total solar reflectance of 99 %, an albedo of 0.96, and a thermal emittance of 0.80. The combination of high albedo and high emittance is enabled by wavelength-selective scattering induced by the hierarchical morphology of nanoPAN, which includes both thin fibers and bead-like structures. During outdoor testing, the material outperforms the radiative cooling power of a state-of-the-art control by ∼20 W/m²and boosts the photocurrent produced by a commercial silicon cell by up to 6.4 mA/cm²compared to sand. These experiments validate essential characteristics of a high-albedo radiative-cooling reflector with promising potential applications in thermal and light management of fielded bifacial panels. 

Silicon-based thermophotovoltaic (TPV) cells offer a promising pathway for clean energy generation leveraging silicon’s abundance, cost-effectiveness, and well-established fabrication processes. Our group has previously demonstrated small area lateral p-n junction Si TPV cells achieving high power conversion efficiency (PCE) = 19% at 1988K employing the air-bridge back surface reflector (BSR) which enables significantly higher out-of-band reflectance (~98%) compared to conventional Au BSR (~95.3%). In this work, we present the successful fabrication and performance characterization of a large-scale Si TPV module integrating the air-bridge BSR. Our module consists of 40 interconnected Si TPV cells (arranged in a 4 × 10 configuration) fabricated on a single 4-inch wafer. We achieve open circuit voltage (VOC) = 4.9 V, short circuit current (ISC) = 0.33 A, fill factor (FF) = 51% and power output (Pmax) = 820 mW at an emitter temperature of 2400°C, confirming the robust performance of air-bridge Si TPV technology at scale. Our results mark a significant step towards the commercialization of Si TPV systems by validating their scalability. 

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.