Near-Field Thermophotovoltaic Conversion with High Electrical Power Density and Cell Efficiency above 14.

A huge amount of thermal energy is available close to material surfaces in radiative and nonradiative states, which can be useful for matter characterization or energy harvesting. Even though a full class of novel nanoengineered devices has been predicted over the last two decades for exploiting near-field thermal photons, efficient near-field thermophotovoltaic conversion could not be achieved experimentally until now. Here, we realize a proof of principle by using a micrometer-sized indium antimonide photovoltaic cell cooled at 77 K and approached at nanometer distances from a hot (∼730 K) graphite microsphere emitter.

Radiative heat transfer at the nanoscale: experimental trends and challenges.

Energy transport theories are being revisited at the nanoscale, as macroscopic laws known for a century are broken at dimensions smaller than those associated with energy carriers. For thermal radiation, where the typical dimension is provided by Wien's wavelength, Planck's law and associated concepts describing surface-to-surface radiative transfer have to be replaced by a full electromagnetic framework capturing near-field radiative heat transfer (photon tunnelling between close bodies), interference effects and sub-wavelength thermal emission (emitting body of small size). It is only during the last decade that nanotechnology has allowed for many experimental verifications - with a recent boom - of the large increase of radiative heat transfer at the nanoscale.

New insights into the thermal behavior and management of thermophotovoltaic systems.

The thermal behavior of a thermophotovoltaic system composed of a metallo-dielectric spectrally selective radiator at high temperature and a GaSb photovoltaic cell in the far field is investigated. Using a coupled radiative, electrical and thermal model, we highlight that, without a large conductive-convective heat transfer coefficient applied to the cell, the rise in temperature of the photovoltaic cell induces dramatic efficiency losses. We then investigate solutions to mitigate thermal effects, such as radiative cooling or the decrease of the emissivity or the temperature of the radiator.

Micron-sized liquid nitrogen-cooled indium antimonide photovoltaic cell for near-field thermophotovoltaics.

Simulations of near-field thermophotovoltaic devices predict promising performance, but experimental observations remain challenging. Having the lowest bandgap among III-V semiconductors, indium antimonide (InSb) is an attractive choice for the photovoltaic cell, provided it is cooled to a low temperature, typically around 77 K. Here, by taking into account fabrication and operating constraints, radiation transfer and low-injection charge transport simulations are made to find the optimum architecture for the photovoltaic cell.

Pathways for mitigating thermal losses in solar photovoltaics.

To improve the performance of solar photovoltaic devices one should mitigate three types of losses: optical, electrical and thermal. However, further reducing the optical and electrical losses in modern photovoltaic devices is becoming increasingly costly. Therefore, there is a rising interest in minimizing the thermal losses.

Spectrally shaping high-temperature radiators for thermophotovoltaics using Mo-HfO trilayer-on-substrate structures.

Easy-to-fabricate, high-temperature, thermally-stable radiators are critical elements for developing efficient and sustainable thermophotovoltaic energy conversion devices. In this frame, a trilayer-on-substrate structure is selected. It is composed of a refractory metal -molybdenum - constituting the substrate and an intermediate thin film sandwiched between two hafnia transparent layers.

High-injection effects in near-field thermophotovoltaic devices.

In near-field thermophotovoltaics, a substantial enhancement of the electrical power output is expected as a result of the larger photogeneration of electron-hole pairs due to the tunneling of evanescent modes from the thermal radiator to the photovoltaic cell. The common low-injection approximation, which considers that the local carrier density due to photogeneration is moderate in comparison to that due to doping, needs therefore to be assessed. By solving the full drift-diffusion equations, the existence of high-injection effects is studied in the case of a GaSb p-on-n junction cell and a radiator supporting surface polaritons.

Spectral and total temperature-dependent emissivities of few-layer structures on a metallic substrate.

We investigate the thermal radiative emission of few-layer structures deposited on a metallic substrate and its dependence on temperature with the Fluctuational Electrodynamics approach. We highlight the impact of the variations of the optical properties of metallic layers on their temperature-dependent emissivity. Fabry-Pérot spectral selection involving at most two transparent layers and one thin reflective layer leads to well-defined peaks and to the amplification of the substrate emission.

Impacts of propagating, frustrated and surface modes on radiative, electrical and thermal losses in nanoscale-gap thermophotovoltaic power generators.

The impacts of radiative, electrical and thermal losses on the performances of nanoscale-gap thermophotovoltaic (nano-TPV) power generators consisting of a gallium antimonide cell paired with a broadband tungsten and a radiatively-optimized Drude radiator are analyzed. Results reveal that surface mode mediated nano-TPV power generation with the Drude radiator outperforms the tungsten radiator, dominated by frustrated modes, only for a vacuum gap thickness of 10 nm and if both electrical and thermal losses are neglected. The key limiting factors for the Drude- and tungsten-based devices are respectively the recombination of electron-hole pairs at the cell surface and thermalization of radiation with energy larger than the cell absorption bandgap.

Polarization effects in 3D vectorial-induced current reconstructions.

In tomography algorithms, the complex amplitude scattering matrix corresponds to the input parameter. When considering 3D targets, the scattering matrix now contains vectorial information. Thus, this scattering matrix might be calculated with various polarization projections.

Microwave measurements of the full amplitude scattering matrix of a complex aggregate: a database for the assessment of light scattering codes.

We present an extensive experimental study of microwave scattering by a fully characterized complex aggregate. We measured the full amplitude scattering matrix (amplitude and phase of the four elements) for a wide range of configurations. The presented results are of special interest to the light scattering community.