Spectral filtering of sub-bandgap radiation using all-dielectric gratings for thermophotovoltaic applications

Abstract

Filtering of the sub-band spectral radiation is an attractive technique to overcome the lower efficiencies of direct conversion thermophotovoltaic technology. The poor performance of these systems is due to the relatively small portion of the incident energy being above the bandgap of photovoltaic cell. To effectively filter the majority of the sub-bandgap radiation and re-employ it as regenerative heat, a viable solution is to design an efficient spectrally selective filter ideally matched to the photovoltaic cell’s bandgap. Here, we have explored a high contrast amorphous silicon grating on a quartz substrate. Quartz due to its inherent nature inhibits transmission of sub-band gap radiation in the infrared (IR) region (>4.75  μm), whereas gratings further filter radiation above 1.8  μm suitable for GaSb photovoltaic cell. The optimized filter is fabricated using direct write laser lithography, and optical characterization result shows that 72% of incident radiation in unconvertible region (>1.80  μm) is filtered and recycled. Further, the thermal characterization results of IR filter carried out using a ceramic heater has shown the drop in effective temperature from 1074.9 to 813.2 K in above bandgap region. This suppressed radiation has contributed to an absolute increase in source body temperature by 16.0 K resulting in increase in the above bandgap radiation available for thermophotovoltaic conversion. The proposed spectral filtering design can be tailored to solar cells of any bandgap and is scalable for employment in various thermophotovoltaic applications.