Theoretically, single-junction photovoltaic solar cells can only achieve 34% efficiency, a theoretical maximum, since that’s pretty much all the spectrum they can absorb and convert to electricity.
Currently, the most-efficient commercially-available solar cells are just % efficient, and some theoretical stacked solar cells up to 45% efficient. This is where thermophotovoltaic solar cells come in, essentially a combination of layers that increase the amount of energy that the photovoltaic cell can capture. Since silicon solar cells only respond to infrared light, the perfect layer to pair with it would be an absorber/emitter layer that converts the rest of the solar spectrum into infrared light. Such a pairing could theoretically lead to an 80%-efficient thermophotovoltaic solar cell, something completely out of reach of current photovoltaic technology.
The absorber layer absorbs solar energy, heating up the emitter layer, which is where things were falling apart for this theory, quite literally. The emitter layer, tungsten, was degrading in the high heat created by the absorber, upwards of 1,800°F. Researchers collaborating between Stanford University, University of Illinois-Urbana Champaign, and North Carolina State University got around this problem by coating the tungsten emitter in a nanolayer of ceramic hafnium dioxide. The new ceramic-coated emitter was able to withstand temperatures of 1,800°F or better for about 12 hours, and over 2,200°F for at least an hour.
The results are encouraging, and could lead to very efficient thermophotovoltaic solar cells not much more expensive than existing technology. Hafnium and tungsten are abundant, and the nanomaterials technology used to manufacture the absorber/emitter layer is well-established. Research into other ceramics is ongoing, and no word on when something like this might be commercially available.
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