Figure 1. Near-field thermophotovoltaic cell.

A hot source (temperature T_s) is placed in front of a cell at temperature T_c, which is typically a p-n junction. The source is heated by an external radiation flow, and the temperature of the cell is kept constant in time. The radiation flux between source and cell is converted into an electric current inside the cell, extracted by means of the two electrodes connected to the junction. In our case, the source (at temperature T_s = 450 K) is made of hexagonal Boron Nitride (hBN), optically described by a Drude-Lorentz model with ε_∞ = 4.88, ω_L = 3.032 × 10¹⁴ rad s⁻¹, ω_R = 2.575 × 10¹⁴ rad s⁻¹ and Γ = 1.001 × 10¹² rad s⁻¹ (Ref. ¹⁴). This model predicts the existence of a surface phonon-polariton resonance at frequency . The cell, having temperature T_c = 300 K, is made of Indium Antimonide (InSb), described in the frequency domain of interest by a dielectric permittivity ε₂(ω) = (n_r(ω) + icα(ω)/2ω)² m where n_r(ω) is the refractive index, α(ω) = 0 for ω < ω_g and for ω > ω_g. The value of the gap frequency and the choice α₀ = 0.7 μm⁻¹ reasonably reproduce the experimental values of absorption²⁷.