MIT Research Could Dramatically Improve Solar Cell Efficiency

By experimenting with the photovoltaic reaction in solar cells, researchers at MIT have made a breakthrough in energy efficiency that significantly pushes the boundaries of current commercial cells on the market.

Electrical engineers at MIT have broken the limit on the number of electrons extracted from a solar cell during a photon incident, which is currently one, and proven that what is called "singlet exciton fission" is possible, they said.

In this reaction, more than one electron can be extracted from a solar cell to ultimately produce more electricity, work that supports an ongoing endeavor to break the so-called Shockley-Queisser (SQ) efficiency limit, which posits that the ultimate conversion efficiency for a single optimized semiconductor junction can never exceed 34 percent, Nicholas Thompson, a graduate student working on the project, told Design News. He is also first author on a report about the work published in Science magazine.

"When a photon is absorbed by a normal solar cell material, it is limited to only supplying one electron to the external circuit," he told us. "What we have shown here is that using a specific material -- an organic semiconductor called pentacene -- we can beat that one photon to one electron limit."

What this means in layperson's terms is that by replicating this reaction in commercial solar cells, which use silicon as their semiconductor, the standard of efficiency -- currently at 25 percent -- could go beyond 30 percent or even more, researchers said.

This would be a huge leap, said Marc Baldo, an MIT electrical engineering professor and principal investigator in the Research Laboratory of Electronics (RLE), in an article on the MIT website. "We showed that we could get through that barrier."

While the basis for the research is not new -- going back theoretically to the 1960s -- MIT's research team is the first to successfully complete a proof of principle of singlet exciton fission. An exciton is the excited state of a molecule after absorbing energy from a photon. Using an organic semiconductor like pentacene -- made from carbon and hydrogen -- in a photovoltaic reaction could even break the 34 percent SQ barrier. This is what researchers used in their experiment to make the reaction occur.

Thompson explained to Design News the difference between pentacene and silicon, and the occurrence of singlet exciton fission in this way:

In silicon, when a blue photon is absorbed, an electron is highly excited. This electron very quickly thermally relaxes, wasting much of its energy as heat. In pentacene, the excited electron transfers some of its energy to another electron (the singlet exciton fission process), both of which can be converted to current. This process loses significantly less energy to heat, allowing for more sunlight to be captured, hence why it is a step-stone toward breaking the Shockley- Queisser limit.

To conduct their experiment, researchers input a known number of photons of a specific color onto a solar cell and then counted the electrons that came out. "When we achieved 1.09 electrons per photon, we knew that we have successfully achieved our goal of using singlet fission to beat the one electron per photon limit," Thompson told us.

There are currently a limited number of organic semiconductors that can do this kind of reaction, so currently, researchers are focusing on coating known singlet fission material on silicon solar cells to achieve more efficiency and push it beyond 30 percent. "This increase could come at little increase in cost, making it a practical and exciting advance in solar technology," Thompson said.

The next step, he added, would be to prove in a laboratory that a singlet fission material like pentacene could be successfully developed as part of a commercial solar cell, but this work is a bit further down the line.

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