By adding a highly reflective mirror on the back of a photovoltaic cell, they broke efficiency records at the time and have continued to do so with subsequent research.
They used a simple gold mirror and adding a dielectric layer above the gold will improve our efficiency to 36 percent.
PNAS – Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering
Significance
Thermophotovoltaic conversion utilizes thermal radiation to generate electricity in a photovoltaic cell. On a solar cell, the addition of a highly reflective rear mirror maximizes the extraction of luminescence, which in turn boosts the voltage. This has enabled the creation of record-breaking solar cells. The rear mirror also reflects low-energy photons back into the emitter, recovering the energy. This radically improves thermophotovoltaic efficiency. Therefore, the luminescence extraction rear mirror serves a dual function; boosting the voltage, and reusing the low-energy thermal photons. Owing to the dual functionality of the rear mirror, we achieve a thermophotovoltaic efficiency of 29.1% at 1,207 °C, a temperature compatible with furnaces, and a new world record at temperatures below 2,000 °C.
Abstract
Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical over 50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.