Hopkins Chemists Develop Efficient Molecular Solar Cells
The new cells, also being developed in labs around the world, produce energy at less than one-tenth the cost of the conventional silicon solar cells. They also are less expensive to manufacture, and recent advances have led to a 10-fold increase in their efficiency at converting sunlight into energy, making them about as efficient as conventional solar cells.
A breakthrough at Johns Hopkins involves the preparation of a new light-absorbing synthetic dye, bonded with another chemical in one "supermolecule" that increases the voltage production by 50 percent over similar cells. Light shining onto the cells "excites" the dye molecules, causing electrons to jump out of the dye and creating "holes" where they used to be. One obstacle in producing efficient molecular-scale solar cells is that electrons tend to quickly refill the holes left by the missing electrons, stopping the flow of current and wasting the energy initially injected into the system by the light.
The goal is to retard that "recombination" of electrons into the holes for as long as possible, prolonging electric current and producing useful energy.
The dye molecule produced by Johns Hopkins chemists slows down the recombination process by at least 1,000 times, compared to previously known dye molecules, said Gerald Meyer, an assistant professor of chemistry, who led a team of scientists making the discovery.
Here is how the molecular solar cell works: small panels of glass are covered with tiny particles of titanium dioxide -- a semiconductor -- and then coated with the dye. When light shines onto the panel, the dye molecules are excited, injecting electrons into the semiconductor and creating an electric current. The key to sustaining the current is the addition of chemicals known as "electron donors," which inject electrons into the holes in the dye, in effect "regenerating" the dye molecules. That prevents the original electrons from recombining with the dye molecule and forces them to combine with the electron donor instead, delaying the recombination process and prolonging the flow of electrical current.
In similar cells, scientists use a salt called sodium iodide as an electron donor. The salt is dissolved in a solution sandwiched between the semiconductor and a platinum electrode. When the dye molecule loses an electron, the iodide quickly contributes one to the dye, turning into iodine in the process. Meanwhile, the electron originally from the dye molecule flows through the semiconductor and into a circuit where it can do useful work, such as lighting a light bulb. Then that electron recombines with the iodine, turning it back into iodide so that the cycle can start over again.
What's new is that the Johns Hopkins scientists have taken the process a step further; they chemically bonded an additional electron donor to the dye molecule itself, producing a supermolecule that performs two functions: absorbing light and injecting an electron into the dye when a hole is created. The additional electron donor is a chemical called phenothiazine.
With two electron donors instead of one, the system's efficiency has been dramatically improved, increasing voltage by more than 50 percent.
The dye molecules act as molecular "electron pumps," Meyer said. They absorb light, inject an electron into the semiconductor, accept an electron from the donor and then start the cycle over again.
The key to heightened efficiency is that, instead of combining with the iodine, the electron in the semiconductor has to combine with the phenothiazine, which is located farther away from the surface of the semiconductor. So, it takes longer. As a result, the recombination process is slowed down dramatically: instead of a millionth of a second, it takes closer to a thousandth of a second for the electron-hole pair to recombine.
Solar cells based on this new compound are about as efficient as silicon solar cells presently on the market, converting roughly 10 percent of sunlight into electricity. A potential advantage of these cells is that they will be cheaper to manufacture than conventional cells, producing electricity at a cost of about 50 cents per kilowatt hour. Traditional photovoltaic cells produce electricity at a cost of $8 to $10 per kilowatt hour. Similar molecular structures might also one day be used in "molecular electronics," for tiny circuits and batteries.
In a figurative sense, the cells work like chlorophyll in plants. Although plants do not produce electrical current from sunlight, they do efficiently produce energy from sunlight at the molecular level by preventing the recombination of electrons and holes.
The cells are not yet ready for practical application because their high efficiency degrades slowly over time. Scientists are working on ways to maintain the high efficiency levels. "The real test now is long-term stability," Meyer said. "Could you make a cell that could sit on a rooftop for a couple of years and not poop out on you, not start to degrade, in terms of its efficiency?"
A scientific paper about the innovation was published Nov. 29 in the Journal of the American Chemical Society. The research is funded by the National Science Foundation and the National Renewable Energy Lab.
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