Abstract
We show theoretically that 2D rectangular gratings on the surface of GaSb can serve as an “anti-reflection” pattern for nano-gap thermophotovoltaic (TPV) devices, which significantly enhances near-field radiative flux from the emitter to a GaSb cell, thus improving output power and conversion efficiency. The system in this study is a 200-nm gap TPV power generation system with a planar infrared plasmonic emitter and GaSb cell. Rigorous coupled-wave analysis is used to calculate the spectral near-field radiative flux involving periodic structures. The simulation shows that when coupled with a near-infrared plasmonic bulk emitter, adding gratings on the GaSb cell surface results in strong spectral enhancement above the cell’s bandgap and suppression for low-energy photon transmission, an effect that cannot be fully predicted by the effective medium theory. The resultant peak spectral heat flux is 2.8 times as high as the case without surface structures and the radiative transfer efficiency increased to 24.8% from the original 14.5% with the emitter temperature at 1800 K. The influence of the grating’s geometry parameters on the enhancement and peak frequency is further discussed with rigorous calculation of the spatial distribution of thermal radiative transfer that provided insight into the physical mechanism.
Highlights
A TPV device generates electricity by converting the photons emitted from the high-temperature emitter through a low-bandgap PV cell, usually composed by group III–V semiconductor materials
The manipulation of near-field spectral radiative flux has been demonstrated in many systems, practical designs aimed at TPV systems are still scarce, mainly because group III–V semiconductors, typically GaSb, used as TPV cells have large refractive indices in the near to mid-IR spectral region and lack any plasmonic properties, which results in high Fresnel reflection coefficients and makes them poor thermal absorbers[8]
As the near-field spectral heat flux is determined by the coupling optical properties of materials on both sides, the weak absorption of GaSb greatly adds to the difficulty of spectral design to improve the output power and efficiency of a near-field TPV system
Summary
A TPV device generates electricity by converting the photons emitted from the high-temperature emitter through a low-bandgap PV cell, usually composed by group III–V semiconductor materials. Chalabi et al.[13,14,15] presented simulations showing that 1D gratings on SiC increased the near-field radiative flux and reduced the far-field radiative transfer These very different radiative transfer behavior characteristics between the near-field and far-field effects are mainly due to the participation of evanescent waves with large lateral wave vectors, which do not exist in far-field scenarios, and due to the path of the radiative transfer becoming comparable to the characteristic size of the microstructures, which means that the addition of surface structures may significantly change the effective gap distances between two objects and alter the spectral and spatial patterns of the radiative transfer for nano-size gaps. The aim of this work is to develop a near-field “anti-reflection” surface structure, which is analogous to its far-field counterparts in suppressing the semiconductor absorber’s reflection and enhancing the heat flux above the cell’s bandgap and remains effective for near-field setups by considering a sub-micron radiative transfer path and the participation of evanescent waves. The results with gratings added are compared to planar setups and the physical mechanisms are explained
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