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. The team 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 nm. 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 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.
"We have proposed a new approach to the use of dirty silicon," said
Eicke Weber, UC Berkeley professor of materials science and engineering,
principal investigator of the Center for Advanced Materials at the
Lawrence Berkeley National Laboratory, and principal investigator of
the research project. "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."
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