Novel Cells and Systems for Intermediate Concentration Photovoltaics

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In the last few years, there have been dramatic decreases in the cost of solar photovoltaics (PV) such that it is now the least expensive and most rapidly growing form of utility-scale electricity supply in many jurisdictions. Most PV energy is provided by silicon modules, where the cell cost is now only a small fraction of the overall system cost, making cell efficiency paramount in driving further reduction in energy cost. While the average efficiency of all PV cells deployed globally is only ~18%, commercial size cells have been demonstrated with an efficiency of 26.7% and deployed cell efficiencies are expected to continue to rise for the next 5-10 years, fundamentally limited to 29.4% by Auger recombination. In an alternate approach called concentrator photovoltaics (CPV), sunlight is focussed ~1000X onto a solar cell by an optical system that accurately tracks the sun. In this approach, very high efficiency multijunction solar cells which have reached a record 47.1% efficiency can be used because their high cost is mitigated by the high level of optical concentration. Despite these benefits, CPV systems remain costly as compared to the much simpler solar modules deployed at scale. In order to accelerate global PV deployment, new strategies at the cell/system level are required to further increase cell efficiency and lower system costs. One avenue that has not been fully explored is consideration of intermediate optical concentrations, in the range of 5-200X, where the main benefit of concentration the use of higher-cost higher-efficiency cells can still be accrued, but the cost and complexity of the concentrator system can be greatly reduced. At the system level, requirements for optical components and tracking systems become less stringent and we will develop novel low profile passive tracking systems, suitable for commercial rooftop applications. At the lowest concentrations (5-10X), non-tracking designs are possible and we will develop high optical-throughput concentrator systems. At the cell level, intermediate concentration makes it feasible to introduce more complex fabrication processes such as photolithography, CVD growth and ion implantation that are currently too costly for 1-sun applications. As one example, we have shown theoretically that use of repeated silicon cells for the base of a tandem cell opens up new material choices for the top cell, while still achieving high efficiency. We will further develop this concept and design/fabricate/test new cell designs for high-efficiency intermediate concentration applications. The anticipated outcome of this research program is a deep and comprehensive understanding of the scientific and technological opportunities of intermediate concentration photovoltaics, providing a long-term conceptual roadmap for future work. Working with Canadian industry, we will develop new high-efficiency and low-cost PV cells and systems for deployment in Canada and for export to the global market.