Solar cells are not perfect; they do not convert all the energy in electromagnetic waves into electricity. Every per cent of improvement to their performance is of significant importance, bringing both economic and environmental benefits to the society. The limitations of solar cell performance are due to the fact that the photovoltaic cell utilizes just part of the solar spectrum, and this part of the spectrum is determined by the band-gap properties of semiconductor material used in the cell. The remaining electromagnetic energy is either reflected or absorbed by the photovoltaic cell and released in the form of thermal heat. The idea behind thermophotovoltaic (TPV) cells is to have the electromagnetic energy first converted to heat, and then re-emitted in a narrow spectral range that is efficiently converted to electricity by a solar cell. While the efficient conversion of light into heat is readily achievable, narrow-spectrum thermal re-emission of electromagnetic waves is quite a challenging problem. Significant research efforts are made around the world to tackle this problem.
The authors of this manuscript propose a method for achieving narrow band thermal emission of photons by using the metamaterial approach, i.e. by designing an artificial material with desired electromagnetic properties. Their key goals are to have narrow band, polarization-insensitive absorption. This is achieved by using engineered metal-dielectric composite structures, which have resonances (“poles”) that ensure narrow band emission of radiation. For successful operation of such a TPV, its temperature due to solar heating should reach values close to 1500K, which would certainly melt many metals. Instead of using classic metals, the authors propose to utilize recently characterised plasmonic materials, such as titanium nitride, which have melting temperature at 3250K. The authors study the performance of TPVs with various materials used in a metamaterial arrangement, and theoretically prove their superior performance as compared to other contemporary designs.
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