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 twojunction
CdZnTe/Si solar cells, concentrating on the value of the open-circuit voltage Voc because it measures the
absorber-material limitations on cell efficiencies. We found Voc ≥ 90% of its thermodynamic limit, equivalent to the best
reported results for single-junction III-V cells.