For decades researchers have investigated a theoretical means to double the power output of solar cells--by making use of so-called "hot electrons." Now researchers at Boston College have provided new experimental evidence that the theory will work. They built solar cells that get a power boost from high-energy photons. This boost, the researchers say, is the result of extracting hot electrons. The results are a step toward solar cells that break conventional efficiency limits. Because of the way ordinary solar cells work, they can, in theory, convert at most about 35 percent of the energy in sunlight into electricity, wasting the rest as heat. Making use of hot electrons could result in efficiencies as high as 67 percent, says Matthew Beard, a senior scientist at the National Renewable Energy Laboratory in Golden, CO, who was not involved in the current work. Doubling the efficiency of solar cells could cut the cost of solar power in half.
Conventional solar cells can only efficiently convert the energy of certain wavelengths of light into electricity. For example, when a solar cell optimized for red wavelengths of light absorbs photons of red light, it produces electrons with energy levels similar to those of the incoming photons. When the cell absorbs a higher-energy blue photon, it first produces a similarly high-energy electron--a hot electron. But this loses much of its energy very quickly as heat before it can escape the cell to produce electricity. (Conversely, cells optimized for blue light don't convert red light into electricity, so they sacrifice the energy in this part of the spectrum.)
The Boston College researchers made ultra-thin solar cells just 15 nanometers thick. Because the cells were so thin, the hot electrons could be pulled out of the cell quickly, before they cooled. The researchers found that the voltage output of the cells increased when they illuminated them with blue light rather than red. "Now we're getting the electrons from the blue light out before they lose all of their excess energy," says Michael Naughton, a professor of physics at Boston College.
The problem is that because they're so thin, the solar cells let most of the incoming light pass through them. As a result, they convert only 3 percent of the energy in incoming light into electricity. "I think it's promising," Beard says. But he adds that so far they're only showing "a pretty small effect."
Naughton says that his team plans to address this problem using nanowires. The basic idea, put forward by many different researchers now, is to make forests of nanowires that will absorb light along their lengths. And because each nanowire is thin, the electrons won't have far to travel to escape to a conductive layer on its surface. This could make it possible to replicate the hot electron effect seen in the thin solar cells. Naughton and colleagues are commercializing such nanowires via a startup called Solasta, based in Newton, MA, which is being funded by the respected venture capital firm Kleiner Perkins Caufield & Byers.
The researchers also hope to increase the number of hot electrons they collect from the absorbed light. To do this, they are turning to an approach taken by Martin Green, a professor at the University of New South Wales in Australia and a leader in using hot electrons in solar cells. This method involves incorporating a layer of quantum dots, which act as a sort-of filter, selectively extracting higher-than-normal-voltage electrons, Beard says. Naughton says that Solasta has already demonstrated that it's possible to incorporate such quantum dots into the company's nanowires.
By Kevin Bullis