With the addition of sensors and enhanced communication tools, providing lightweight, portable power has become more challenging. Researchers have now demonstrated a new approach to turning thermal energy into electricity that could provide compact and efficient power for soldiers on future battlefields.

Hot objects radiate light in the form of photons into their surroundings. The emitted photons can be captured by a photovoltaic cell and converted to useful electric energy. This approach to energy conversion is called far-field thermophotovoltaics (FF-TPVs) and has been under development for many years; however, it suffers from low power density, and therefore, requires high operating temperatures of the emitter.

In the new approach, the separation between the emitter and the photovoltaic cell is reduced to the nanoscale, enabling much greater power output than what is possible with FF-TPVs for the same emitter temperature. It enables capture of energy that is otherwise trapped in the near-field of the emitter — called near-field thermo-photovoltaics (NF-TPV) — and uses custom-built photovoltaic cells and emitter designs for near-field operating conditions.

The technique exhibited a power density almost an order of magnitude higher than that for the best-reported near-field-TPV systems, while also operating at six times higher efficiency, paving the way for future near-field-TPV applications. In the future, near-field-TPVs could serve as more compact and higher-efficiency power sources for soldiers, as these devices can function at lower operating temperatures than conventional TPVs.

The efficiency of a TPV device is characterized by how much of the total energy transfer between the emitter and the photovoltaic cell is used to excite the electron-hole pairs in the photovoltaic cell. While increasing the temperature of the emitter increases the number of photons above the bandgap of the cell, the number of sub-bandgap photons that can heat up the photovoltaic cell need to be minimized.

This was achieved by fabricating thin-film TPV cells with ultra-flat surfaces and with a metal back reflector. The photons above the bandgap of the cell are efficiently absorbed in the micron-thick semiconductor, while those below the bandgap are reflected back to the silicon emitter and recycled.

The researchers grew thin-film indium gallium arsenide photovoltaic cells on thick semiconductor substrates and then peeled off the very thin semiconductor active region of the cell and transferred it to a silicon substrate. The researchers also performed theoretical calculations to estimate the performance of the photovoltaic cell at each temperature and gap size and showed good agreement between the experiments and computational predictions.

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