Researchers Poke Holes in Solar Cells to Improve Efficiency
Hao-Chih Yaun, a researcher at the National Renewable Energy Laboratory, pours a mixture of hydrofluoric acid and hydrogen peroxide onto a silicon wafer as part of the process to create a dark solar cell. (Source: Dennis Schroeder/NREL)
This is indeed an optical effect. In conventional cells the surface is texturized, the idea being that small angled facets on the surface of a cell trap light inside once it has gotten there or is secondary emission produced by carrier recombination. For monocrystalline cells the texture is in the form of little pyramids while for multicrystaline cells it is more like the surface of a sponge. In either case, this is coated with an antireflective coating which, when done well already makes the cells pretty black. There have been many attempts to improve on this; however, the common process provides a second function which is to passivate the textured surface of the cell which is done during the process of depositing the anti-reflective coat. The limitiation for other methods has been that nothing else has provided the two functions at a lower cost, keeping in mind that the remaining ground to be gained is only 4 or 5% in air and less in a solar module. The reflectance of bare cells is not the issue as the cells are laminated to glazing and the reflections at the boundaries between air and glass, glass and encapsulant, encapsulant and AR coat are all important. The reflectance of an encapsulated cell can be reduced just as easily by increasing the refractive index of the encapsulant or by reducing reflection at the front surface of the glazing. In a typical solar module, the solar gain of an encapsulated cell is higher than it is in free air.
This approach uses nano-structure to make the shift in refractive index at the surface less distinct although air entrapment during lamination could actually reduce the benefit. Also, since a portion of photons are reflected by the back surface of the cell and, hopefully, the front surface of the cell after that (i.e. photons are trapped), one has to be careful not to negate internal reflection at the front surface of the cell.
This is a great idea and it may be a real benefit, but there is clearly a trade-off in that if nothing is reflected then the heat absorbtion will also increase a bit, which means that some method of dealing with the higher operating temperature will be needed. So thee is an unintended consequence, although what the ultimate effect is will need to be studied a bit more. Or possibly the result of hotter operation is a better output, I am not aware of any recent published data about the effects, although years ago heat was not of any benefit to solar cell operation.
That's the key, isn't it, Nancy? I know other research efforts that are working to drive down the cost of production, too. There should be some changes to this market coming very soon that will help drive adoption of solar on a larger scale, I think.
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