Can a Solar Cell Ever Be 100% Efficient?

I’d like to know if it is theoretically possible to design a solar cell with a 100% efficiency. — S.T., Montreal, Canada

The experts say “no.” Because there are specific spectral ranges for semiconductor materials, some of the radiant solar energy just cannot be used. Researchers are working with stacking various materials into multi-junction cells to gather solar energy in many ranges, but there are still going to be losses when some of the photon energy becomes heat rather than energy, as well as optical losses, electrical resistance losses and other aspects of the photovoltaic (PV) process that prevents total use of the available solar energy. However, there have been consistent increases in PV efficiencies for the past 30 years and it seems that every year researchers are finding ways to make PV cells that convert more of the photon energy of light into the electron energy of electricity. Right now, there’s a lot of excitement since the announcement a few months ago that a multi-junction cell developed by Boeing-Spectrolab had achieved a sunlight-to-electricity conversion ratio of just over 40 percent. That’s a huge leap over the top efficiencies today that range from around 13 to 24 percent in the lab, depending on the material used and the solar concentration ratio (production efficiencies are around 7 to 17 percent). Developing cells with significantly higher efficiencies offers the greatest promise for big drops in the cost of PV systems. Maybe the best clue as to what the upper limits might be comes from a DARPA program being conducted at the University of Delaware and a consortium of 15 members to double the efficiency of cells by 2010. Their goal is to reach around 54 percent efficiency in the laboratory, which would equal around 50 percent in actual production. There have been other theories offered in recent years that have projected possible huge increases in efficiency. NREL scientists, for example, are looking at nanocrystals called quantum dots that they think could reach high efficiencies of 50 percent or more. A group called RSL Energy announced a few months ago that they were working on a semiconductor material using indium and gallium and hoped to reach an efficiency of 48 percent. Researchers at the Technical University of Munich are working on using gold particles on silicon to improve efficiencies by 15 to 20 percent. If PV is ever going to be cost-competitive with other sources of electricity, the costs must drop considerably. Improving the cell efficiency is a major step in making this happen. Ken Sheinkopf is Director of Public Affairs at the Florida Solar Energy Center (FSEC) and Vice President of The Sheinkopf Group
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