Near-field radiative absorption enhancement induced by anisotropic medium back surface layer in thermophotovoltaics

The near-field thermophotovoltaic (NFTPV) system with a backside reflector and an air-bridge has been proven to be effective in reducing parasitic loss and improving efficiency. However, these structures are limited in enhancing energy absorption of the cell and improving output performance. In this work, based on the concept of medium-bridge NFTPV system, we introduce a uniaxial anisotropic medium back surface layer between the cell and the reflector and study its impact on the energy absorption of the cell and the energy transfer in the system. We find that elliptical electromagnetic mode in the medium back surface layer is more conducive to promoting the performance, challenging the general view that hyperbolic materials always play a crucial role in energy transfer. Due to the elliptical electromagnetic mode in the medium back surface layer, the heat flux absorbed by the InAs cell above the bandgap can reach 9.32 × 10⁵ W/m², which is about 8.6 times larger than that of the system applying a normal air-bridge, and about 2.2 times larger than that of the hyperbolic case. Finally, we investigate the influence of the imaginary part of the medium back surface layer on energy transfer in the system and prove the robustness of the enhancement effect when considering optical loss of the medium back surface layer. Our research reveals a paradigm for uniaxial anisotropic medium back surface layer NFTPV systems, and brings significant enhancement to the energy absorption of the cell.