The cells with the highest efficiency need pure silicon. When impurities are present in the silicon crystal they have a tendency to capture electrons that are needed to generate the electricity.
By Sarah Yang
BERKELEY - A research team led by engineers at the University of California, Berkeley, has developed a new technique to handle metal defects in low-grade silicon, an advance that could dramatically reduce the cost of solar cells.
Nearly 90 percent of solar, or photovoltaic, cells in the world are made from a refined, highly purified form of silicon, the same material used to make integrated circuits. The growth of the semiconductor and solar cell industries has put increasing pressure on relatively limited supplies of this high-quality silicon, consequently driving up the price of the material.
"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 Weber. "The current worldwide growth rate of photovoltaics is 30 to 45 percent 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."
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 nanometers.
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.
The researchers point out that techniques such as varying the cooling rate of silicon is an easy, cost-effective adjustment to current manufacturing procedures.
"We're targeting mainstream technology," said Weber. "The approach we are proposing could lead to substantial progress in making solar energy more widely available with just a few tweaks in the manufacturing process."
The researchers say that by 2006, the photovoltaic industry is projected to use more silicon than the microelectronic industry, and that keeping solar energy cost-effective may depend upon finding ways to utilize the dirtier, cheaper silicon material.
The findings were published Aug. 14 in the journal Nature Materials. The link is to the press release from UC Berkeley.
There may be other detrimental effects using the "dirty" silicon that have not been discovered yet, but this is a promising development.
I know this is a couple of months old, but didn't see it posted. Thought it fit in well with the ongoing energy discussion.