USING SUNLIGHT MORE EFFICIENTLY

Researchers at the National Renewable Energy Laboratory (NREL) in Golden, Colo have developed a way for low-cost solar cells to more efficiently convert sunlight into electricity.The research, which increases the "lifetime" of electrons created in a solar cell so they can make more electricity, is a possible step in the direction of bringing down the relatively high cost of solar cells. Reducing cost while sustaining efficiency is the big factor in determining how soon solar power will become a major player in the energy business. Generally you could have good efficiency or low cost but not both. Efficiency refers to the fraction of the sunlight falling on the solar panel that actually gets converted into useable electricity. And cost refers to the expense of mass-producing the panels in large sheets. Solar cells have been used in niche markets, such as for powering remote sensors or spacecraft, and are increasingly used for homes and utility applications.

Most of these solar cells are made from crystalline silicon. But for large-scale adoption to occur, the price will have to come down. Currently the cost-per-kilowatt-hour for solar-generated power is several times higher than for generating that power with fossil fuels. Solar cells mimic nature in the way that it converts sunlight into useful energy. In a green leaf, for example, the incoming sunlight liberates an electron in a molecule of chlorophyll. The electron (and its energy) gets passed from one molecule, eventually being incorporated into building up larger molecules such as a carbohydrate. In a solar cell the incoming sunlight liberates an electron from a piece of semiconductor. This "excited" electron, if it stays excited, can be incorporated into an electrical current feeding into an external circuit, where it can flow into a battery or the electric grid. The longer the lifetime of the excited electron, the better the efficiency of the solar cell. Unfortunately, electrons tend to lose their energy when they meet a defect or boundary in the crystals that make up a solar cell. Until now to get a better excitation lifetime and better efficiency, solar cells needed to be made of higher-priced single crystal materials like silicon or gallium arsenide. These solar cells need lots of complex processing to build, and these costs are not likely to be reduced. Meanwhile, lower-priced solar cells made from thin layers of multi-crystalline materials, such as compounds made of the atoms copper, indium, gallium, and selenium (CIGS), haven't been nearly as efficient.

The research focused on improving electron lifetimes in solar cells made from multi-crystalline CIGS, and in their research paper, NREL scientists Wyatt Metzger, Ingrid Repins, and Miguel Contreras announced they have achieved an electron lifetime of 250 billionths of a second. It sounds like a short time, but it is long enough for more electrons to contribute to the cell's electricity, making it dramatically more efficient, yet still low in cost when compared to the high-efficiency silicon solar cells. The results were recently published in the journal Applied Physics Letters.

(Phillip F. Schewe)