MIT research looking for cheaper, more abundant solar cell materials

By Rick C. Hodgin

Boston (MA) - Research teams at MIT are taking a new look at solar energy. They're trying to figure out a way to use the Earth's most abundant elements to create inexpensive solar grids capable of generating terawatts of power. The idea is simple: Make solar cells more affordable using readily available materials and the market will naturally migrate over.


30-500-10

The team's research is well underway - even heading toward commercial application. They began by choosing 30 of the Earth's most abundant elements as their starting point. They then worked backwards from the 500 or so compounds those elements are typically found in. They wanted to figure out which of those compounds could be used to harness solar energy. In the end, about 10 promising compounds were selected for detailed study, which is currently ongoing.

The first material chosen was cuprous oxide (Cu₂O), a reddish mineral often found in pigments and fungicides. According to Tonio Buonassisi, assistant professor on the project, "[Cuprous oxide] is promising, the optical properties are just right. But the electrical properties are not up to snuff." The team has been attempting to improve its electrical properties with defect engineering methods (those which inject anomalies into the material which alter its electrical properties). At the same time, the remaining candidate compounds are being examined.


Return on investment

According to an interview with Jerry Woodall, Purdue professor with expertise in solar technologies, the break-even point for any kind of alternate energy source (including solar) is 15% efficiency. According to Woodall, no matter how inexpensive it is to produce, if it's not at least 15% efficient, then it's not worth the effort - for a commercial endeavor that is.

Still, the key goal of MIT's research is to reduce production costs of the material itself. If the team can find a suitable replacement for the expensive "highly purified silicon," which is used to create traditional solar cells today, then the effort will have paid off.


Multi-crystalline form

The team has also examined how to replace the expensive single-crystal silicon used today with the much less expensive multi-crystalline silicon. They've been looking to find a correct temperature profile for a re-reheating the silicon after it is initially grown and cooled, a process called annealing.

Multi-crystalline silicon is not very efficient for use in solar cells as its many defects absorb the converted energy. The team has discovered that annealing can greatly reduce the defect's absorption rate, even by a hundred-fold, making multi-crystalline a possible inexpensive alternative without switching compounds.

The report also notes that much of this research is already behind them and production is in advanced stages. Full details of these findings appeared in a Fall issue of Applied Physics Letters. And currently, the team is already working with manufactures to bring products based on their work to the market. According to the report, "Pilot runs are expected within a year [by the end of 2009], and full-scale production soon thereafter." If true, inexpensive multi-crystalline silicon-based solar cells could be available to the public in 2010.


Previous solar research

MIT has been focused on solar cell research for many years. Their report includes citations for scientists dating back to the 1950s. And, as recently as late 2007, MIT scientists began creating a commercial endeavor to pursue a previous invention, called string ribbon solar cells. They received $12.4 million in funding this past March to continue their string ribbon effort. Additional significant advances in reflective materials, and solar cells with a special coating applied which enable them to receive 90% of the sun's energy without moving to track the sun's arc through the sky.

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