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.
The quest for the development of portable thermophotovoltaic (TPV) systems has been a growing interest due to the ability to achieve high power and energy densities using hydrocarbon based fuels. Recent studies based on intermediate filters and photonic crystals have shown significant improvement in system efficiencies for combustion driven and solar-based TPV systems. The key goal is to engineer directionally and spectrally selective thermal emitters ideally matched to the solar cell. Here, a high contrast grating based thermal emitter using silicon as a grating material on a quartz substrate is proposed which is suitable for integrating to GaSb solar cell based thermophotovoltaic systems powered by microcombustor. The intrinsic properties of quartz substrate filter the below bandgap (greater than 4.5 μm) radiation in the infrared region. The silicon gratings are optimized (period = 2.4 μm, duty cycle = 40 % and thickness = 0.55 μm), to provide transmission only for photons with wavelengths lower than 1.8 μm thus inhibiting below bandgap radiation of GaSb cell. The spectrally tuned emitter structure shows transmission of more than 70% of convertible photons (above the bandgap) and reflection of 80% of unconvertible photons (below bandgap) back to the combustor thus reducing the heat losses in the photovoltaic conversion and increasing the combustion system temperature there by contributing to overall increase in TPV system efficiency.