Designing High-Efficiency Thermophotovoltaics by Recycling Below-Bandgap Photons Through High Reflectivity



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Thermalphotovoltaic (TPV) cells turn thermal radiation from an emitting heat source to generate electricity in a photovoltaic cell. In TPVs, the above-band gap photons (~0.354 eV) from a thermal emitter excite electronic transitions in the photovoltaic cell and photogenerated charge carriers are subsequently separated and extracted as electricity. The below-bandgap photons, however, are lost as heat. The motivation to further expand this field stems from the possibility of spectral control for recovery of unconverted energy in the below-bandgap frequency range. Traditional approach is to use back surface reflectors on the backside of photovoltaic cell to reflect the unused photon to the heat source. But reflectivity is still not reaching the theoretical limit (100%). In this work, we propose to study high performance mirror designs that can potentially out-perform traditional mirrors. The proposed model builds on the work of Dr. Zhao, et al [1] where an ITO emitter and InAs cell were used for waste heat recovery applications and is considered as a benchmark for studying mirror performance. We first consider inserting an air cavity in between the InAs cell and a metal mirror, which is proposed by Ref. [2] as an effective approach to improve reflectivity. We then replace the air gap with multilayer structures to see whether a better reflectivity can be achieved by optimizing the multilayer materials and geometries. The overall efficiency improvement with our design will be quantified with optoelectronic simulations with the focus being on designing the back mirror for InAs cell.



Mechanical Engineering