By Phillip F. Schewe
ISNS Contributor
October 1, 2008
Researchers at the National Renewable Energy Laboratory in Golden,
Colorado, 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. The results were recently published in
the journal Applied Physics Letters.
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.
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