Direct heat-to-electricity conversion is crucial for advanced thermal energy technologies. Thermophotovoltaic (TPV) devices, comprising a hot emitter and a photovoltaic (PV) cell, are being explored for this purpose. In TPVs, radiation from the hot body generates electricity in the PV cell via the photovoltaic effect. Performance is characterized by efficiency (electrical power output/radiative heat transfer) and power density (electrical power output/unit area). Far-field TPVs, with emitter-cell separations exceeding thermal wavelengths, have achieved efficiencies up to 30% but are limited by the Stefan-Boltzmann law. Near-field (NF) TPVs overcome this by using nanoscale gaps, enabling evanescent modes to dominate energy transfer. While NF effects have been predicted and demonstrated, experimental NFTPV systems have been limited by challenges in creating high-temperature emitters and high-quality PV cells suitable for selective absorption of near-field thermal radiation while maintaining precise nanoscale gap control. Previous NFTPV demonstrations showed low efficiencies (<0.1%) and power densities (~6 W/m²), highlighting the need for improved device design and fabrication.

Existing literature extensively explores the theoretical potential of near-field thermophotovoltaics, predicting high efficiency and power density through the exploitation of evanescent modes in nanoscale gaps. Several computational studies have modeled different emitter and absorber materials and geometries, suggesting significant enhancements over far-field systems. However, experimental verification has lagged behind theoretical predictions, with earlier demonstrations exhibiting limited efficiency and power density. Previous studies using silicon emitters and InAs or InGaAs cells showed improvements over far-field systems but remained far below the theoretical potential. The use of cryogenically cooled cells in some studies, while showing high cell efficiency, yielded low overall efficiencies due to the energy required for cooling. This gap between theory and experiment highlights the significant challenges in fabricating thermally robust emitters and high-performance PV cells compatible with near-field operation.

This research developed microdevices capable of reaching temperatures up to 1270 K and matching thin-film InGaAs PV cells with spectral response optimized for selective absorption of above-bandgap thermal radiation. The emitter is a monolithic, doped silicon cantilever with a circular mesa heated via Joule heating. A 10 nm AIN layer provides electrical insulation and protects the emitter surface at high temperatures. The PV cell has a circular active area (190 µm diameter) closely matching the emitter dimensions and features a thin-film In0.53Ga0.47As layer epitaxially grown on InP, transferred to a silicon substrate, with top and bottom Au layers serving as electrical contacts, with the bottom contact acting as a back surface reflector. The emitter and PV cell were precisely aligned using a nanopositioning platform in a high-vacuum environment. The gap size was systematically controlled, ranging from micrometers to nanometers, even at high emitter temperatures. Electrical power output was measured using current-voltage (I-V) characterization at each gap size. A model based on fluctuational electrodynamics was developed to estimate power output and total radiative heat transfer. The model considered the emitter's mesa and recessed ring regions, accounting for both near-field and far-field contributions. Emitter temperature was measured using scanning thermal probe microscopy. The spectral energy transfer was calculated to understand the near-field enhancement mechanisms. The efficiency (η) was determined as the ratio of measured power output (PMPP) to the calculated total radiative heat transfer (QRHT).

The study achieved record power densities of ~5 kW/m² at an efficiency of 6.8% at an emitter temperature of 1270 K with a 100 nm gap. This represents a significant improvement compared to previously reported near-field TPV devices. Near-field enhancements were observed, with an 8-fold increase in power output at 70 nm gap compared to the far-field (7 µm). This enhancement was attributed to the increased above-bandgap (ABG) photon flux from evanescent modes coupled at sub-wavelength gaps. The model accurately predicted the experimental power output. The efficiency increased with emitter temperature and was significantly greater than previous reports at temperatures above 930 K. The spectral energy transfer analysis showed significant enhancement in ABG energy transfer at 100 nm gap, while maintaining suppression of sub-bandgap (SBG) energy transfer. The transmission coefficient analysis showed the significant contribution of evanescent modes to near-field enhancement. The short-circuit current (Isc) and open-circuit voltage (Voc) increased with temperature and near-field enhancement. The power density showed a clear enhancement in the near-field at all temperatures, with efficiency slightly lower in the near-field due to absorption in the Au reflector.

The results demonstrate the successful realization of efficient and high-power density near-field thermophotovoltaic energy conversion. The achieved power density is more than an order of magnitude higher than the best-reported near-field TPV systems, while operating at significantly higher efficiency. The close agreement between experimental data and the theoretical model confirms the understanding of the underlying physical mechanisms driving the near-field enhancement. The observed increase in efficiency with temperature highlights the potential for significant improvement in near-field TPV technology, offering a viable route towards waste heat recovery applications.

This study demonstrated highly efficient near-field thermophotovoltaic energy conversion at record power densities. The observed near-field enhancements and the agreement between experiment and theory provide valuable insights into the fundamental principles of near-field thermal energy harvesting. Future work should focus on improving the long-term stability of high-temperature emitters, exploring alternative materials and designs to further enhance efficiency, and addressing the SBG absorption in the Au reflector. The successful demonstration of this technology paves the way for advanced energy conversion applications.

The current study focuses on planar structures. Real-world applications might require adaptation to different geometries, potentially influencing the efficiency. The model used some simplifying assumptions (e.g., infinite lateral extent of the devices) which could influence the accuracy of the calculated heat transfer and efficiency. Long-term stability of the devices at high temperatures under various operating conditions still needs further investigation. The use of a gold back reflector in the PV cell contributes to some SBG energy transfer that could be further reduced. Further, slight disagreement between the experimental and theoretical efficiency at the highest temperatures might be due to uncertainties in temperature measurements or model parameters.