The theoretical limit of solar energy conversion, called the Shockley-Queisser Limit, was set in 1961. Ever since, engineers have been pushing the boundaries and reaching for higher conversion rates. Single-layer silicon solar cells, the most common variety, have an upper limit around 32 percent efficiency. Using multiple layers of solar cells, or multipliers like mirrors, has been the only effective way to exceed that upper limit for some time. Last week, we reported a development by a team of Australian researchers, who set a new world record for solar efficiency with a prismatic solar cell that converts 34 percent of direct sunlight into electricity without multipliers. MIT’s new solar cell could boost that figure even higher.
The MIT team—doctoral student David Bierman, professors Evelyn Wang and Marin Soljačić, and four others—took the other path toward higher efficiency, by working to convert sunlight first into heat, and then into electricity, using devices known as solar thermophotovoltaics (STPVs). Acknowledging the previously held ideas about limitations of solar efficiency, Bierman said, “with solar thermal photovoltaics you have the possibility to exceed that.” And they did.
MIT’s ‘hot new solar cell’ successfully demonstrated how STPVs are an improvement over traditional low-efficiency PV cells, although their invention isn’t grabbing any world records. “We have demonstrated, for the first time, an STPV device that has a higher solar-to-electrical conversion efficiency compared to that of the underlying PV cell,” said Wang. The team’s demonstration had an overall efficiency of only 6.8 percent, but that was a clear improvement in comparison to the original PV cell operating on its own. The study was published in the journal Nature Energy.
