Traditional photovoltaic cell materials
Traditional silicon-based PV cells have an operational efficiency of only 20%, which means they convert only a fifth of the sun’s energy falling on them into electricity. This because of silicon’s physical properties – it cannot do anything with the photons from the shorter-wavelength violet, indigo, blue, and green components of light.
Higher operating efficiencies of more than 50% are indeed possible with other semiconductor materials. Gallium phosphide and gallium arsenide can be stacked one above the other to capture various components of the same light, but this technology is expensive. As such, they are used only in one-off applications like space satellites, where efficiency matters more than cost.
New photovoltaic cell with step-like design
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A team of researchers from MIT and the Masdar Institute of Science and Technology, a graduate-level research university in the United Arab Emirates that focuses on sustainability, may have found a way to balance efficiency with cost concerns. Their photovoltaic cell has two layers, but the lower layer juts out from beneath the upper layer, resulting in a step-like design. They call it the ‘step cell’. This kind of ‘junction’ technology (where two layers are joined to each other) normally comes at a premium, but the team looked hard to ways to bring down the cost. They finally settled down on a substrate – the material that the layers are placed on – that could be reused.
Explaining the rationale behind the design, Dr. Eugene Fitzgerald, MIT Professor of Materials Science and Engineering, says the two layers are not really joined together, but bonded. With ‘joined’ technology, if one layer becomes defective/inefficient, it alone cannot be replaced, as it is joined to the other layer. The entire cell has to be discarded. With the new bonded technology, any layer can be replaced as and when it becomes defective or inefficient – there is no need to junk the entire cell and get a new one. This improves the life of each cell.
The reason for the step design is so that the top layer of gallium arsenide phosphide can capture the shorter wavelength components of light, while the silicon layer underneath works with the longer wavelength red component of light. The ‘step’ prevents wastage that would occur if they were placed side-by-side. Part of the silicon layer is below the gallium arsenide phosphide, so this absorbs light that trickles down. Placing the silicon below the gallium arsenide phosphide would mean imply more time taken, so this revolutionary approach does away with both possible disadvantages.
Theoretical efficiency is 40%, but practical efficiencies observed are as high as 35%, according to Dr. Ammar Nayfeh, Associate Professor of Electrical Engineering and Computer Science at Masdar Institute of Science and Technology. The step cell could be put into commercial production within 1-2 years, and even if there are no takers, the United Arab Emirates, which funds research at the Masdar Institute, could be its biggest patron. The oil-rich emirate of Abu Dhabi, which is where the university is located, has taken a cue from its neighbor Dubai and started preparing for the day when it runs out of oil. Concentrated Solar Power is another area of research it is interested in, and the International Renewable Energy Agency has its global headquarters right here in Masdar City, powered entirely by solar energy.
Their findings have been published in the IEEE Journal of Photovoltaics and the Journal of Applied Physics. Ms. Sabina Abdul Hadi, the Ph.D. scholar whose research was fundamental to the project, was lauded for her pioneering work at the 42nd IEEE Photovoltaic Specialists Conference in New Orleans, USA. comment