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Harnessing the phenomena could greatly improve yields

In the never-ending quest to improve solar cell efficiencies, researchers have made another thin film breakthrough.  Using an ultra-thin amorphous silicon solar film which is mere nanometers thick, the researchers observed the "hot electron" effect for the first time and were able to harvest its energy, opening the door to efficiency improvements.

Hot electrons are held as somewhat of a “Holy Grail” in the solar cell research community and reportedly will form the basis of so-called "Third Generation" cells.  When light strikes a solar cell, it produces electrons in a variety of energy states.  Energy from mid-range electrons is commonly harvested, but the high-energy hot electrons typically lose most of their high kinetic energy to heat before they reach the conduction band.

The new ultra-thin film -- less than 30 nm thick -- allows hot electrons to instead use an "escape hatch" and avoid losing their energy.  The team, led by Michael J. Naughton, the Evelyn J. and Robert A. Ferris Professor of Physics at Boston College, focused on minimizing the routes in the environment that electrons can escape to.

While the quantum physics behind this endeavor is complex, the paper's lead author Professor of Physics Krzysztof Kemp says that what they were doing was basically going from trying to heat a pool with a pot of boiling water, to a more reasonable task of trying to heat a sink with boiling water.  He elaborates, "We have shrunk the size of the solar cell by making it thin. In doing so, we are bringing these hot electrons closer to the surface, so they can be collected more readily. These electrons have to be captured in less than a picosecond, which is less than one trillionth of a second."

Typical cells commercial photovoltaics offer efficiencies of somewhere between 10 to 20 percent, but are expensive to produce.  The new cells used a mere fiftieth of the thickness and still retained 3 percent efficiency, thanks mainly to harvesting the hot electrons.  Capturing the hot electrons had the extra benefit of reducing waste heat, which saps voltage.

By combining the technology with nanostructures, such as nanowires, the researchers predict that much higher efficiencies can be achieved.  The end will result will likely be much cheaper cells with efficiencies similar to today's, ultimately lowering costs.

Other members of the research team included Professor of Physics Zhifeng Ren, Research Associate Professor and Laboratory Director Andrzej A. Herczynski, Research Scientist Yantao Gao, doctoral student Timothy Kirkpatrick, and Jakub Rybczynski of Solasta Corp., of Newton MA, which supported the research. 

The research is published in the journal Applied Physics Letters.  The abstract can be viewed here.





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