Here we show polymer solar cells manufactured using only printing and coating of abundant materials directly on flexible plastic substrates or barrier foil using only roll-to-roll methods. Central to the development is a particular roll-to-roll compatible post-processing step that converts the pristine and non-functional multilayer-coated stack into a functional solar cell through formation of a charge selective interface, in situ, following a short electrical pulse with a high current density. After the fast post-processing step the device stack becomes active and all devices are functional with a technical yield and consistency that is compelling.