Multijunction single-crystal CdTe-based solar cells: Opportunities and challenges

Solar energy is the only renewable energy source that could largely replace the burning of fossil fuels and is the most rapidly deployable energy source, but because of its high cost, makes up only 1% of the world's energy production. Concentrated photovoltaics (CPV) requires the least land and by far the least semiconductor material of any photovoltaic technology, but is the most expensive. Its high cost arises from the need to approximately lattice match the substrate and III-V materials in the three-junction CPV solar cells to maximize minority-carrier recombination times and hence cell efficiencies. Lattice-matching forces the use of Ge substrates, which are very expensive and fragile, making the cells very expensive. We give experimental evidence and theoretical arguments that, unlike III-V cells, CdTe-based multijunction cells need not be lattice-matched and could be grown on Si by high-throughput molecular beam epitaxy, reducing the cost an order of magnitude. That would allow the use of much lower solar concentrations, greatly reducing the tracking and optics costs. Also, efficiency calculations, assuming lattice matching not to be required for II-VI materials, indicate that the highest-efficiency three-junction II-VI cells should have efficiencies 3-8% (absolute) higher than those of the highest-efficiency three-junction III-V cells. We have fabricated and tested single-junction and two-junction CdZnTe/Si solar cells, concentrating on the value of the open-circuit voltage V-oc because it measures the absorber-material limitations on cell efficiencies. We found V-oc >= 90% of its thermodynamic limit, equivalent to the best reported results for single-junction III-V cells.