By Darius Snieckus
Tuesday, June 03 2014
Each p-n junction generates electric current from a distinct wavelength of incoming sunlight, so the more junctions, the higher the efficiency.
Conventional single-junction designs have a theoretical efficiency ceiling of 34%. Multi-junction cells, assuming no limit to the number of interfaces, could reach efficiencies of 87% under highly concentrated sunlight.
Best lab results for traditional PV cells have shown efficiencies around 25%, whereas for multi-junction cells, the efficiency is pushing towards double that figure.
Last year, researchers at the US National Renewable Energy Laboratory reached a 31.1% conversion efficiency for a two-junction cell.
German R&D giant Fraunhofer ISE, together with developer Soitec, CEA-Leti and the Helmholtz Center Berlin, meanwhile, set a new world record of 44.7% efficiency using a cell with a four-junction architecture.
The current record for a mass-produced PV module is held by US CPV technology developer Semprius and cell supplier Solar Junction, which have a unit that achieved a 35.5% efficiency.
Most multi-junction cells fabricated to date use three layers of semiconductor “tuned” to absorb different light wavelengths. Gallium arsenide (GaAs) compounds are commonly used. The layers are “grown” in descending order of their “bandgap” — the amount of energy needed to kick a semiconductor crystal into action.
Growing layer upon layer of semiconductor material to create a cell can lead to lower quality crystal structures — and so a less efficient cell. The highest performing multi-junction cells have therefore been experimenting with new techniques to connect semiconductor layers to boost efficiency. Fraunhhofer ISE is pioneering a procedure called “wafer bonding”, while Semprius/Solar Junction is advancing a micro-transfer printing technique to knit together layers of the cell.
Researchers are currently targeting a 50% efficiency cell milestone.
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