The efficiency of modern solar cells—the percentage of sunlight converted to electricity— is only around 30%. The rest is lost as heat dissipation following energetic photon-absorption. This is the reason that the maximum Shockley-Queisser efficiency limit is only 41% at the maximum solar illumination level. This limit is the maximum theoretical efficiency of single P-N junction cells.
For many years researchers have tried to attenuate or even eliminate this heat loss by physical mechanisms such as down-conversion of high-energy photons, hot-carrier cells, multi-junction cells, and others. None of these methods have been successful. Only multi-junction cells have yielded an ultra-high efficiency of around 46%. A more radical approach has been to treat solar radiation as a photon source and also as a heat source. This method takes advantage of both the light power and the heat power produced. In this photothermal process, solar thermo-photovoltaics (STPV) convert the heat flux to a thermal emission by using a special absorber/emitter material and it is then absorbed by a matching PV cell. This method produces much greater conversion efficiency, but at a vey high price: the absorber has to be maintained at a temperature above 2,000° Kelvin. At much lower temperatures this method hardly yields an efficiency over 3.2%.
But recently, a team of researchers at the Technion-Israel Institute of Technology developed a method to improve solar cell efficiency by almost 70%. This might be a major breakthrough in overcoming the limitations of actual methods. The team, headed by Professor Carmel Rotschild, developed a photoluminescent material (PL) in which both photonic and thermal excitations are generated. When the material absorbs solar radiation it converts the heat and light into an “ideal” radiation, which in turn impinges on the PV cell and enables higher conversion efficiency. Testing found that the cell’s efficiency improved to more than 50%.
"Solar radiation, on its way to the photovoltaic cells, hits a dedicated material that we developed for this purpose, and the material is heated by the unused part of the spectrum," says Ph.D candidate Assaf Manor. "In addition, the solar radiation in the optimal spectrum is absorbed and re-emitted at a blue-shifted spectrum. This radiation is then harvested by the solar cell, and both the heat and the light are converted to electricity."
The full paper, published October 20 in the online version of Nature Communication, can be found here: http://www.nature.com/articles/ncomms13167