Midsummer makes CIGS (copper indium gallium selenide) solar cells, and it collaborated with University of California, Los Angeles and Pioneer Materials to stack a perovskite cell on top of one of its products to make the tandem cell – with the wide-bandgap perovskite designed to absorb and converting the shorter wavelengths that the now underlying CIGS cell cannot make use of.
This is a four-terminal cell, with mid-stack electrodes that allow the two sub-cells to operate at different currents – which gets the best out of tandem cells.
