A new six-junction solar cell, developed by NREL (National Renewable Energy Laboratory), converts 47.1% of incident light into electricity when combined with optical concentration. Indications thus far suggest solar cells of this type can reach an efficiency rate of 50%.
There are a number of obstacles to commercialization, though NREL believes that will probably be overcome in the near future. One of these is the presence of a resistive barrier inside the cell, which prevents the flow of a high percentage of current. This problem does not allow to achieve a 50% efficiency. Another obstacle to consider is the high cost for the production of the materials needed for the devices.
One way to lower costs is to reduce the active lighting area involved. You could, for example, use a mirror or concentrator to capture light and concentrate it on a specific point.
Solar concentration could reduce the amount of photosensitive material needed by as much as a factor of a hundred or even a thousand. It is well known that efficiency increases when light is concentrated. Prior to this, four-junction solar cells have demonstrated the highest solar conversion efficiency levels, but now, with the adoption of six junctions, the results have greatly improved. A further reduction in series resistance within this structure could realistically allow efficiency levels in excess of 50%.
Single-junction flat-plate terrestrial solar cells are fundamentally limited to about 30% solar-to-electricity conversion efficiency, but multiple junctions and concentrated light make much higher efficiencies practically achievable. Until now, four-junction III–V concentrator solar cells have demonstrated the highest solar conversion efficiencies. Here, we demonstrate 47.1% solar conversion efficiency using a monolithic, series-connected, six-junction inverted metamorphic structure operated under the direct spectrum at 143 Suns concentration. When tuned to the global spectrum, a variation of this structure achieves a 1-Sun global efficiency of 39.2%. Nearly optimal bandgaps for six junctions were fabricated using alloys of III–V semiconductors. To develop these junctions, it was necessary to minimize threading dislocations in lattice-mismatched III–V alloys, prevent phase segregation in metastable quaternary III–V alloys and understand dopant diffusion in complex structures. Further reduction of the series resistance within this structure could realistically enable efficiencies over 50%.
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