The Shockley-Queisser efficiency limit asserts that the ultimate solar cell conversion efficiency can never exceed 34% for a single optimized semiconductor junction. Researchers at MIT have shown they have a way to blow past that limit. That will raise a cheer to an industry in real need of a major boost.
Decades of research on solar cells have considered Shockley-Queisser an absolute limit to the efficiency of such devices in converting sunlight into electricity. The MIT work published last week in a report in the journal Science, co-authored by graduate students including Daniel Congreve, Nicholas Thompson, Eric Hontz and Shane Yost, alumna Jiye Lee ’12, and professor of electrical engineering Marc Baldo and Troy Van Voorhis.
Professor Baldo explains the principle behind the barrier-busting technique has been known theoretically since the 1960s. But it was a somewhat obscure idea that nobody had succeeded in putting into practice. The MIT team was able, for the first time, to perform a successful “proof of principle” of the idea, which is known as singlet exciton fission. (An exciton is the excited state of a molecule after absorbing energy from a photon.)
In a standard photovoltaic (PV) cell, each photon knocks loose exactly one electron inside the PV material. The loose electron then can be harnessed through wires to provide an electrical current.
MIT’s new technique has each photon can knocking two electrons loose. This makes the process much more efficient: In a standard cell, any excess energy carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.
While others have previously “split” a photon’s energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth’s surface. The MIT team’s work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels.
The team uses an organic compound called pentacene in an organic solar cell in a kind of “coating” for a conventional optical trapping scheme. While that material’s ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.
Professor Baldo, who is also the director of the Center for Excitonics, sponsored by the U.S. Department of Energy said, “Our whole project was directed at showing that this splitting process was effective. We showed that we could get through that barrier.”
Congreve pointed out the theoretical basis for this work was laid long ago, but nobody had been able to realize it in a real, functioning system. “In this system everyone knew you could, they were just waiting for someone to do it,” he said.
A third party commentary was provided by Richard Friend, the Cavendish Professor of Physics at the University of Cambridge who said, “This is the landmark event we had all been waiting to see. This is really great research.”
Its way early in the research, the team has just cleared proof of principle and hasn’t yet optimized the energy-conversion efficiency of the system, which remains less than 2 percent. But ratcheting up that efficiency through further optimization should be a straightforward process, the researchers say. “There appears to be no fundamental barrier,” offered Thompson.
Baldo notes today’s commercial solar panels typically have an efficiency of at most 25 percent, a silicon solar cell harnessing singlet fission should make it feasible to achieve efficiency of more than 30 percent. Baldo noted commercialization would be a huge leap in a field typically marked by slow, incremental progress. In solar cell research, he noted, people are striving “for an increase of a tenth of a percent.”
There is also the effect that solar panel efficiencies can also be improved by stacking different solar cells together, but combining solar cells is expensive with conventional solar-cell materials. The new technology instead promises to work as an inexpensive coating on solar cells.
With proof of principle at hand with a known material the team is now exploring new materials that might perform the same trick even better. “The field is working on materials that were chanced upon,” Baldo said. But now that the principles are better understood, researchers can begin exploring possible alternatives in a more systematic way.
Another third party comment comes from Christopher Bardeen, a professor of chemistry at the University of California at Riverside who called the work “very important” and says the process used by the MIT team “represents a first step towards incorporating an exotic photophysical process (fission) into a real device. This achievement will help convince workers in the field that this process has real potential for boosting organic solar cell efficiencies by 25 percent or more.”
Huzzahs all around it seems and well earned too. MIT has filed for a provisional patent on the technology.
Stay on target there at MIT, the photovoltaic industry really needs a breakout to keep and gain market traction.