New optical coating could extend lifetimes of solar cells

U.S. researchers have developed a new class of optical coatings that could make PV cells last six times longer than usual, which in turn could reduce the cost of replacing worn-out cells.

The project is a collaboration between scientists at Case Western Reserve University (CWRU) in Ohio and the University of Rochester in New York.

Optical coatings are layers of material deposited on a surface to either reflect or filter light into different wavelengths. They are an integral part of almost every optical instrument, from eyeglasses to telescopes.

The new coatings can be made to fully reflect a narrow wavelength that corresponds to the range that efficiently produces electricity by a PV cell. That means the new coatings could lead to a high-performance, low-cost solution for solar energy generation, said Giuseppe Strangi, a physics professor at CWRU.

The PV problem is twofold. Solar panels produce energy efficiently during the day, when energy demand is relatively low, but not after sunset, when demand remains high. And storing energy generated by PV panels can be expensive.

Strangi and his collaborators created a novel coating made from a 15- nanometer-thick film of germanium (similar to silicon or tin) that can simultaneously reflect and transmit the same wavelength, or color, and do both with a high degree of clarity.

The researchers call this new class of coatings Fano Resonance Optical Coatings (FROCs), in a nod to Ugo Fano, an Italian-born American physicist who worked with nuclear scientist Enrico Fermi.

Shaped resonances – essentially, a phenomenon in which two light waves are superimposed – are the foundation of light wave transport, and also the development of this new class of optical coatings.

The new coatings, in addition to their ability to be fine-tuned to reflect the perfect PV wavelength, can also be made to absorb the rest of the solar spectrum. That means they could efficiently transfer those wavelengths to heat. And storing that thermal energy is relatively inexpensive, compared to storing electricity.

Such an approach would also protect the PV cell from overheating, leading to the promise of a sixfold increase in the lifespan of PV cells, Strangi said. However, the solar PV energy application in this new process of manipulating wave-optics at the nanoscale will take some time to develop, the scientists noted. New advancements in optical coatings also could lead to other applications, the researchers added. They recently described their findings in Nature Nanotechnology.