  We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS)
monolithic thin-film tandem solar cell grown on ultra-thin (30-microns thick),
light-weight, and flexible polyimide foil with a steady-state power conversion
efficiency of 13.2% and a high open-circuit voltage over 1.75 V under standard
test condition.
