Silicon with metal contaminants is generally not considered for use in solar cells due to its poor performance, even though it is considerably more abundant than "ultra-pure" silicon. Manufacturing techniques used to remove defects are prohibitively expensive, forcing the solar industry to ignore a vast potential resource of silicon.
The University of California team, by using synchrotron x-ray microprobes to analyse the contaminants in silicon, have pioneered a technique that leaves the impurities in but manipulates them so that they do not provide such an impediment to the efficiency of solar cells.
The researchers found that nano-sized defects in the silicon limited the average distance electrons were able to travel before losing their energy, with the longer the distance the greater the energy conversion efficiency of the material.
By varying the cooling rate of the silicon, the researchers found they could affect the distribution of the metal impurities. When cooled quickly, the metal defects were scattered randomly, but when the cooling rate was slowed, the impurities settled into large clusters.
"Using this cooling technique, we were able to improve the distance electrons could travel by a factor of four compared with dirty silicon that had been left unaltered," said team member Tonio Buonassisi.
"Although this is still not as efficient as ultra-pure silicon, it is the proof of principle that poor quality silicon can be easily improved. We are now looking at other techniques that could further enhance the efficiency of dirty silicon."
With the move of solar into mainstream power generation, demand has seen silicon price increases of up to 800%, a cost that transfers to the consumer and blunts uptake of the technology. The team hopes that by further refining its techniques and making "dirty" silicon a viable production material, solar technology can continue to grow.
"Solar energy is often touted as the most promising and secure alternative energy source, capable of reducing our dependence on foreign fuels while reducing the emission of dangerous gases that harm world climate," said lead project investigator Eicke Weber.
"The current worldwide growth rate of photovoltaics is 30-45% per year, which is nothing short of amazing. However, the solar energy industry could grow much faster if researchers and manufacturers could further reduce the cost of solar cells."
