Researchers reported advances in molecular systems for efficient solar energy conversion and storage at the recent American Chemical Society meeting in Boston (Massachusetts, U.S.A.).
(Top) A titanium-oxide solar cell with a manganese-based photosensitizer oxidizes isopropanol to create oxygen, electrons and hydrogen ions. (Bottom) These are recombined with a platinum catalyst.
Researchers reported advances in molecular systems for efficient solar energy conversion and storage at the recent American Chemical Society meeting in Boston (Massachusetts, U.S.A.). Session organizers Victor Batista and Charles Schmuttenmaer from Yale University said, "The world leaders in this field are here."
Much of the work shown at the ACS meeting focused on investigating and optimizing one of more of the fundamental steps in dye-sensitized solar cells, including charge injection, charge collection and regeneration of the chromophore.
Schmuttenmaer emphasized that important recent results include the development of tools to gain insight at the molecular level, including new theoretical as well as spectroscopic and electrochemical methods. He said, "You can get a reaction, but you don't know what's going on until you analyze the individual steps," which allows researchers to optimize the devices.
Several presentations focused on quantum dots, which are promising components of solar cells because they are robust and may offer high absorption of visible and infrared light, where the solar spectrum has maximum intensity. In addition, much work focuses on extended dye molecules that sensitize semiconductors for absorption in the visible and IR. Research on infrared-sensitized solar cells, which could be deposited on transparent windows, was presented by Elena Jakubikova from North Carolina State University (U.S.A.).
A number of papers related to the evolution of multiexcitonic generation. Victor Klimov and R.D. Schaller of Los Alamos National Laboratory (U.S.A.) reported the exciting news that a single photon could generate up to seven excitons and, thus, multiple excited electrons (Phys. Rev. Lett. 92, 186601; Nano Lett. 6, 424). This offered a potential path to increased efficiency of solar cells. Experimental work since then, however, has reduced expectations.
Klimov reported spectroscopic evidence suggesting that two electrons might be generated by a single photon. "It looks like there might be multiple excited electrons," Batista explained. "If so, the outstanding challenge would be to collect them."
Efficiencies for both organic thin-film and dye-sensitized solar cells continue to be well below those of single-crystal silicon (around 20 percent) or commercially available cadmium-telluride solar panels (18.5 percent). Several talks focused on ways to deal with recombination problems, which have limited organic thin film solar cells to efficiencies under 6 percent. Dye-sensitized solar cells, which were also discussed, are typically based on titanium dioxide and offer current efficiencies just above 11 percent.
A number of researchers—including the session organizers—are developing molecular systems that use light to directly power chemical reactions. This strategy bypasses limitations of battery storage by converting solar energy into fuel.
Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics.