Wired to improve

New silicon-based solar cells  

By Ishita Das
Published: Thursday 15 April 2010

imageWITH the Indian government’s pledge of generating 20 gigawatts of power from solar energy by 2020, about US $20 billion must be invested in developing infrastructure. Everyone wants to know which technology gets the largest piece of the cake.

Most agree that as far as solar cells go, the ones made from silicon are the longest-lasting and most reliable. The deal-breaker for the 50-year-old technology is its very high cost compared to power from coal. Scientists from the California Institute of Technology (CalTech) in USA may have found a way around that with a new design.

The design is based on vertical arrays or assembly of silicon wires. These wires are about 1 micron in diameter, thinner than a strand of hair and are embedded on a flexible polymer— polydimethyl siloxane (PDMS). The design achieves 96 per cent absorption of photons at a single wavelength and 85 per cent of total sunlight that falls on them. Also, the team used hundredth of silicon per square metre of a solar cell. The absorption efficiency of these cells is very close to commercially produced crystalline silicon cells. It even reduces the high costs associated with silicon.

“We have surpassed previous optical microstructures developed to trap light,” said Harry Atwater, professor of applied physics and mathematics and director of Resnick Institute at Caltech. An antireflective coating of silicon nitride was applied to the PDMS following which nanoparticles of aluminium oxide were added. The antireflective coating did what the name suggests and the nanoparticles incorporated in the polymer acted as reflectors so light could bounce back towards the wires increasing chances of getting absorbed. “By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration,“ said Atwater.

The article appeared in the February 14 issue of Nature Materials. The use of PDMS and thin wires makes these cells flexible and lightweight unlike the traditional crystalline silicon cells that are heavy and brittle. For the first time these cells can compete with conductive polymer-based organic solar cells known for their ease of use and commercially available in handbags and umbrellas.

The drawback, however, is that only three per cent of solar energy is converted to electricity in the new design; commercially available silicon-based solar cells are 17 per cent efficient. The new design also uses liquid electrolyte in its batteries. “Considering their low efficiency and use of liquid electrolytes to make electrical contact, commercial production of these cells is a practical problem,” said Erik Garnett, a researcher at Stanford University in USA.

Garnett worked on enhancing the performance of solar cells. The difference between his design and Atwaters’ is that Garnett uses silicon wires 10 times thinner and shorter, present on a silicon substrate (not PDMS). He does not use liquid electrolyte.

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