We report on inverted polymer tandem solar cells wherein the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), modified at one interface with ethoxylated polyethylenimine (PEIE), acts as an efficient charge recombination layer. This recombination layer shows very low optical absorption, high electrical conductivity, and a large work function contrast of 1.3 eV between its top and bottom interfaces. Its use yields tandem cells in which the open-circuit voltage is the sum of that of individual cells. The fill factor of tandem cells connected in series is found to be larger than that of single-junction cells. Its simple polymeric composition and its unprecedented performance make it a promising component for emerging organic photovoltaic technologies.