Researchers push solar cell efficiency to 35%
A new solar cell configuration developed by engineers at the University of New South Wales has pushed the efficiency of solar cells to 34.5%, a new world record for unfocused sunlight.
The experimental set up uses a 28cm2 four-junction mini-module embedded in a prism to extracts the maximum energy from sunlight. It does this by splitting the incoming rays into four bands, using a hybrid four-junction receiver to absorb different frequencies in the separate layers.
The UNSW result, confirmed by the US National Renewable Energy Laboratory, is almost 44% better than the previous record – made by Alta Devices of the USA, which reached 24% efficiency, but over a larger surface area of 800cm2.
“This encouraging result shows that there are still advances to come in photovoltaics research to make solar cells even more efficient,” said Dr Mark Keevers, Senior Research Fellow of UNSW’s Australian Centre for Advanced Photovoltaics. “Extracting more energy from every beam of sunlight is critical to reducing the cost of electricity generated by solar cells as it lowers the investment needed, and delivering payback faster.”
A recent study by Germany’s Agora Energiewende think tank set an aggressive target of 35% efficiency by 2050 for a module that uses unconcentrated sunlight, such as the standard ones on family homes.
The triple-junction cell targets discrete bands of the incoming sunlight, using a combination of three layers: indium-gallium-phosphide; indium-gallium-arsenide; and germanium. As sunlight passes through each layer, energy is extracted by each junction at its most efficient wavelength, while the unused part of the light passes through to the next layer, and so on. Some of the infrared band of incoming sunlight, unused by the triple-junction cell, is filtered out and bounced onto the silicon cell, thereby extracting just about all of the energy from each beam of sunlight hitting the mini-module.
Scaling the cell up to a larger 800cm2 is well within reach, says Keevers. “There’ll be some marginal loss from interconnection in the scale-up, but we are so far ahead that it’s entirely feasible,” he said. The theoretical limit for such a four-junction device is thought to be 53%, which puts the UNSW result two-thirds of the way there.
Multi-junction solar cells of this type are unlikely to find their way onto the rooftops of homes and offices soon, as they require more effort to manufacture and therefore cost more than standard crystalline silicon cells with a single junction. But the UNSW team is working on new techniques to reduce the manufacturing complexity, and create cheaper multi-junction cells. However, the spectrum-splitting approach can be used for solar towers that use mirrors to concentrate sunlight which is then converted directly into electricity.
Other research partners working with the UNSW researchers include Trina Solar, a PV module manufacturer, and the US National Renewable Energy Laboratory.
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