Thanks to a new coating developed at the Massachusetts Institute of Technology (MIT), solar cells could produce two electrons for every particle of light harvested at the green and blue wavelengths. The research advance could be the key to solar cell efficiencies beyond the Shockley-Queisser limit, which proposes that the ultimate conversion efficiency can never exceed 34% for a single optimised semiconductor junction.
The MIT researchers use the organic molecule pentacene to demonstrate singlet exciton fission, where two electrons are produced for each photon, instead of one. “The physics of what happens to an excited electron — or exciton — in this molecule is different from that of a typical semiconductor such as silicon,” says MIT engineering graduate student Nicholas J. Thompson, who is also a first author of the paper “External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission-Based Organic Photovoltaic Cell,” published in Science. Typically, when a blue or green photon is absorbed and an electron is exited, much of its energy is lost in form of heat. “In our singlet fission device, the excited electron transfers some of its energy to another electron, both of which can be collected as current,” Thompson explains. “This process loses significantly less energy to heat, allowing for more sunlight to be captured and a more efficient cell.”
Thompson agrees that this technology applied to conventional solar cells could ultimately be a game-changer for solar energy. The performance of current single-band-gap silicon solar cells tops out at about 25% efficiency in a lab setting. “By doubling the electrons generated from the green and blue photons, we expect that the solar cell efficiency could theoretically exceed 30%,” he says. “This increase would come at little increase in cost, making it a practical and exciting advance in solar technology.”
The MIT team “expressly chose” the organic molecule pentacene in an organic solar cell “because it was well suited to helping us demonstrate our goal of showing that the system can produce more than one electron per photon in the visible portion of the solar spectrum,” Thompson says. The prototype system was only 1.8% power efficient, mainly due to low energy at which the electron leaves the system, as the researcher reasoned. “A second consequence of using an organic solar cell is that we do not capture all the low-energy photons that we should,” he says, adding that these photons would be captured in a solar cell where the singlet fission material is coated onto a conventional solar cell, as opposed to a next-generation organic solar cell.
Several issues remain to be resolved before the technology of singlet exciton fission can be brought to the marketplace. A major milestone will now be to actually use a singlet fission material to improve the power conversion efficiency of a conventional solar cell, such as a silicon cell. “If this is achieved in the laboratory, then we imagine the solar industry will consider that the technology is commercially viable,” Thompson anticipates. He says, “An ideally designed system should perform singlet fission in the blue and green, and absorb normally in the red and infrared.”
Thompson and his colleagues are currently attempting to apply singlet exciton fission to existing high-efficiency silicon solar cells, experimenting with several methods.