Nearly 90 percent of solar, or photovoltaic, cells are made from a refined and purified form of silicon. Attempts to use the far more abundant and cheaper form of silicon one that is laden with metal impurities and defects have failed because solar cells made from this material do not perform as well.
In addition, manufacturing techniques used to remove impurities are expensive, negating the cost benefits of using the cheaper material.
"We have proposed a new approach to the use of dirty silicon," said Eicke Weber, professor of materials science and engineering at the University of California at Berkeley and principal investigator of the Center for Advanced Materials at the Lawrence Berkeley National Laboratory (LBNL), in a statement.
"Instead of taking the impurities out, we can leave them in but manipulate them in a way that reduces their detrimental impact on the solar cell efficiency," Weber said.
The researchers say the findings, published Aug. 14 in the journal Nature Materials, could reduce the cost of solar cells by making the use of cheaper materials feasible. The solar energy industry could grow much faster if researchers and manufacturers could further reduce the cost of the cells, according to the researchers.
“Researchers analyzed how metal contaminants in silicon respond to different types of processing using highly sensitive synchrotron X-ray microprobes capable of detecting metal clusters as small as 30 nanometers,” according to the university.
“The researchers found that the nano-sized defects scattered throughout the silicon limited the average distance electrons were able to travel before losing their energy,” the university said. “The longer the distance, known as the minority carrier diffusion length, the greater the energy conversion efficiency of the material.”
The researchers found that they were able to manipulate the distribution of the metal impurities by varying the cooling rate of the silicon. When the material is cooled quickly, the metal defects are quickly locked in a scattered distribution. By simply slowing down the cooling rate, the metal impurities diffused 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 Tonio Buonassisi, a Ph.D. student in materials science and engineering at the university. "Although this is still not as efficient as ultrapure 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."