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Breakthrough in solar photovoltaics

THE HOLY Grail of researchers in the field of solar photovoltaic (SPV) electricity is to generate it at a lower cost than that of grid electricity. The goal now seems to be within reach.

A Palo Alto (California ) start-up, named Nanosolar Inc., founded in 2002, claims that it has developed a commercial scale technology that can deliver solar electricity at 5 cents per kilowatt-hour.

Molecular self-assembly

The breakthrough has come through the application of nanotechnology to create components via molecular self-assembly, including quantum dots (10nm large nanoparticles) as well as nanotemplates with structural order extending through all three dimensions.

In addition, Nanosolar has demonstrated that the three dimensionally engineered nanotemplates can be conformally coated or solidly filled with semiconductor paint to create ultra-thin solar cells with layers that are yet another factor 100x thinner than conventional thin-film amorphous silicon solar cells.

This allows a 10x larger surface area of these structures to be used to achieve a 10x increase in efficiency for such thin layers, thus making it possible to use even less material for similarly efficient cells. Conventional inorganic semiconductors tend to require intricate processing to ensure large grains of crystallinity (in the extreme case: mono-crystallinity) so that charges can travel hundreds of nanometres without getting trapped and lost (at internal crystal boundaries).

The 3D nanocomposite architecture of the ultra-thin-absorber cells makes possible absorption of a substantial fraction of the incoming sunlight despite the ultra-thin layers since the charges need to be transported only several nanometres without much opportunity for a loss.

This means the requirements on the semiconductor material can be relaxed and low cost materials such as inorganic semiconductors of the IIb/VIa and Ib/IIIa/VIa families as well as solution-coatable organic semiconductors can be used.

Lower cost

According to the CEO, Martin Roscheisen, the conversion efficiency (percentage of incident light energy converted to electrical energy) of the Nanosolar SPV cell is above 12 per cent for its first product prototypes. He claims that the Nanosolar SPV cell costs only $ 0.36 per peak watt.

The semiconductor paint can be applied to a flexible substrate , such as a polymer sheet , through a simple web printing process, to create an array of ultra-thin solar cells.

Nanosolar has developed proprietary substrate technology that keeps the substrate cost within a smaller fraction of the overall product cost than any other state-of-the-art thin-film solar cell technology. The company has also developed a powerful new way of interconnecting individual solar cells into larger modules and large-area sheets and allows high-throughput module assembly at high yield.

The flagship product, Nanosolar SolarPly, is a 14 feet x 10 feet solar electricity module delivering 120 watts per square inch at 110V. The company is now offering solar panels at below $1 per peak watt.

The Nanosolar team, headed by CEO Martin Roscheisen (listed by Fortune in 2003 among the top ten U.S. entrepreneurs below 40 years of age), has some top-notch Indian technologists assisting it.

Among them are Dr. Siva Sivaram (ex-Intel) and Dr. Arati Prabhakar , former Director of the U.S. National Institute of Standards and Technology.

N.N. Sachitanand

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