BERKELEY, CA. -- Common manufacturing methods produce solar cells with anefficiency of 12 to 15 percent in converting sunlight to electricity; to make aprofit, 14 percent is the bare minimum. In work done at the Ernest OrlandoLawrence Berkeley National Laboratory, scientist Scott McHugo has discoveredimportant clues to the poor performance of solar cells manufactured frompolycrystalline silicon.
The solar-cell market is potentially vast; because there's no need tobuild transmission lines or truck in generator fuel, lightweight solar panelsare ideal for bringing electrical power to remote locations in developingnations. Industrial nations faced with diminishing resources also have activeprograms aimed at producing better, cheaper solar cells.
"In a solar cell there's a junction between p-type silicon and an n-typelayer such as diffused-in phosphorus. When sunlight is absorbed, it freeselectrons which start migrating in a random-walk fashion toward that junction,"explains McHugo; now with Berkeley Lab's Accelerator and Fusion ResearchDivision, McHugo became interested in solar cells when he was doing graduatework in materials science at UC Berkeley. "If the electrons make it to thejunction, they contribute to the cell's output of electric current. Often,however, before they reach the junction they recombine at specific sites in thecrystal," and thus can't contribute to current output. McHugo looked at a map of a silicon wafer in which sites of highrecombination showed up as dark regions. Researchers before him had shown thatthese occurred not primarily at grain boundaries in the polycrystallinematerial, as might be expected, but more often at dislocations in the crystal --yet the dislocations themselves were not the problem. Using a unique heattreatment, McHugo performed electrical measurements to investigate the materialat the dislocations; he was the first to show that they were "decorated" withiron.
"When I came to Berkeley Lab as a postdoc, I was able to employ atechnique using x-rays at the Advanced Light Source (ALS) which is orders ofmagnitude better than what can be done with standard techniques that use anelectron beam," says McHugo, who worked with the x-ray fluorescence microprobebeamline built and operated at the ALS by the Center for X-Ray Optics, part ofthe Lab's Materials Science Division. The one-micron spot of hard x-raysproduced by the beamline allowed McHugo to align the resulting x-rayfluorescence spectra with maps of the defects made with a scanning electronmicroscope, comparing defects and impurities directly. "That's when I found thatnot only iron but copper and nickel were also concentrated in thesehigh-recombination sites."
Metal from valves, couplings, and other machinery can contaminate solarcells as they are grown from molten silicon, cut into wafers, and finished byadding dopants and attaching contacts. In an industry with a narrow profitmargin, where cheap polycrystalline silicon must be used instead ofeasy-to-purify but far more costly single-crystal silicon, rigorous cleanlinessat every step of the way would be too expensive.
However, when it comes to purifying solar cells, cleanliness is not theonly variable. Doping with phosphorus, as well as sintering aluminum contactsonto the wafers (heating them almost to melting), both actually help in"gettering" the silicon -- getting out the contaminants chemically. By adjustingtime and temperature, these standard processes could be optimized to do a betterjob. McHugo has shown that briefly annealing the finished solar cell at hightemperatures is enough to remove copper and nickel precipitates of moderatesize, although dissolved copper and nickel or very small precipitates of thesemetals may remain. McHugo is currently investigating what techniques are necessary toremove stubborn iron impurities from their hiding places in crystal defects."We're looking at a two-step process," he says, "first subjecting the wafer tovery high temperatures and then lowering the temperature to finish the properprocessing of the solar cell."
"If a dirty manufacturing run produces solar cells of 12 percentefficiency, and a manufacturer can make money at 15 percent, think howprofitable cells of 18 percent would be," says McHugo, who has collaborated withAmerican and Japanese manufacturers and is now working with a consortium ofgovernment, university, and industry researchers in Germany. "Investigators have already achieved 18 percent in the lab with small samples; the challenge isto do it on the production line with full-sized solar cell wafers. It's a goalwe're close to reaching."
Some of McHugo's findings were presented at the Materials ResearchSociety meeting held last spring in San Francisco and will appear in theOctober, 1997 issue of Applied Physics Letters.
Berkeley Lab is a U.S. Department of Energy national laboratory locatedin Berkeley, California. It conducts unclassified scientific research and ismanaged by the University of California.
Materials provided by Lawrence Berkeley National Laboratory. Note: Content may be edited for style and length.
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