Optimum selective emitters for efficient thermophotovoltaic conversion

Though thermophotovoltaic (TPV) systems have been studied for many decades, the demonstrated conversion efficiencies have remained far lower than the theoretical maximum. Here, in this work, we investigate the reason for low efficiency, especially in TPV systems employing selective thermal emitters, and determine design pathways toward high efficiency. We model both the optical and optoelectronic components of the TPV system and study the influence of the emitter selectivity on the optimum bandgap of the photovoltaic cell, heat sink requirements, and maximum conversion efficiency for any given emitter temperature from 1000 to 2000 K. Our calculations suggest that thermal emitters with at least 20 dB suppression of sub-bandgap emission and an emission enhancement of 100x can push the overall efficiency to 70% of Carnot's limit. Furthermore, we show that such an extreme requirement on suppression is at the performance limits for resonant thermal emitters employing refractory plasmonic materials such as Mo, W, Ta, TiN, and carbon nanotubes.