Optical analyses of lossy near-field thermophotonic devices with planar and scattering mirrors

Recovering ubiquitous low grade waste heat can lower the amount of global net energy generation required and would thereby support the transition to renewable energy sources. Thermophotonic devices can potentially achieve this, and therefore a coupled transfer matrix-radiative transfer model is presented that offers insight in their optical behavior. In these devices, typically a LED and photodiode (PD) are thermally isolated, but optically coupled by a nanogap to promote near-field energy transfer. The device is sandwiched between mirrors to prevent optical losses, and employed to harvest waste heat from a heat bath connected to the LED. Numerical studies on thermophotonic devices have so far only considered highly idealized cases. The model employed in this work is therefore specifically set up to take into account several loss mechanisms simultaneously, and is computationally light to allow for broad parameter sweeps. Crucially, it still accounts for near-field effects in the nanogap. The performance metrics obtained emphasize that a narrow nanogap and simultaneously a high LED internal radiative efficiency and high mirror quality are required for output powers in excess of 103 W/cm2. Furthermore, it is shown that LED mirror texturization can increase the net output power threefold for devices with experimentally obtainable quality and even more for poor quality devices. Lastly, it is shown that the commonly proposed bandgap alignment between the LED and PD can reduce net power output up to five-fold when the sub-bandgap emission is large, and therefore a reduction in the PD bandgap energy is proposed.