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Copper indium gallium di-selenide [Cu(InGa)Se2or CIGS] thin-film solar cell has attracted great attention because of their high efficiency, low cost potential, less raw materials consumption, and so on. Using polyimide (PI) as the flexible substrate, the CIGS thin-film solar cell has the advantages of light weight, flexibility, and low energy consumption compared with the conventional hard glass substrate. However, there are still challenges in PI foils for flexible CIGS substrate application: (1) the thermal resistance of the PI foils is not high enough, which cannot stand in the high temperature CIGS absorber formation process; and (2) the coefficient of thermal expansion of the PI foils is too large, which causes peeling-off problems of subsequently deposited Mo back electrode on the PI foils. This article reviews the current status of the CIGS solar cells on flexible PI. In addition, recent progress and future prospects of the high temperature resistance and low thermal expansion of PI foils for high efficiency are reviewed.