Near-Field Thermophotovoltaics

by Bong Jae Lee, KAIST, Republic of Korea

A thermophotovoltaic (TPV) cell is an infrared-sensitive photovoltaic cell which can directly convert absorbed infrared radiation into electricity and has potential to outperform other solid-state electricity-generation techniques, such as thermoelectric and thermionic converters. Also, the fact that a TPV system has no moving parts and is independent of pressure and gravitational force makes it attractive for space, industrial, residential and microelectronic applications. The current major challenges in practical use of a TPV system are relatively low electrical power output density (of the order of a W/cm2) while conversion efficiency has recently grown up to 30% (single junction cell) and 40% (dual junction cell). In order to increase the power density, the concept of near-field thermal radiation has been applied to TPV. If the emitter is placed very close to the TPV cell (the so-called near-field TPV configuration), the emitter can transfer more radiative energy to the TPV cell, leading to increased electrical power output, as briefly explained in the following.

In the far field, the blackbody radiation limits the radiative heat transfer from an emitter to a TPV cell. The incidence angle should be smaller than the critical angle for the electromagnetic wave to reach the TPV cell in the far field. In this case, the blackbody radiation is the maximum value of radiation by this general propagative mode. When the incidence angle is greater than the critical angle, the electromagnetic wave in the vacuum is confined to the interface. Such a confined wave, called ‘evanescent wave,’ can also contribute to the radiative heat transfer via photon tunneling when the gap between the emitter and the cell is smaller than the thermal characteristic wavelength determined by Wien’s displacement law (around 1 micron at 1000 K). Due to the additional radiative heat transfer provided by the evanescent waves, thermal radiation can exceed the blackbody radiation. Furthermore, surface polaritons can also contribute to photon tunneling. Metallic materials support surface plasmon polaritons by collective electronic movements, and dielectric materials support surface phonon polaritons by excited transverse optical phonons. The frustrated and surface modes together are called the evanescent mode.

Three modes of near-field thermal radiation; propagative, frustrated, and surface modes.

More information is available in a review article on near-field thermophotovoltaics: Song et al., Solar Energy Materials & Solar Cells 208, 111556 (2022).