We demonstrate a simple and novel technique to improve the power conversion efficiency of polymer:fullerene solar cells based on the low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and the soluble fullerene derivative PCBM. We dope the blend via cosolution with the electron accepting molecule tetrafluoro-tetracyanoquinodimethane
(F4-TCNQ). Doping concentrations of F4-CNQ from 0 - 0.3 %, with respect to the weight of the polymer, were
investigated. The dark current-voltage (IV) characteristics, the illuminated IV characteristics, the external quantum efficiency, and the hole mobility in dependence of the doping concentration were studied. We observe
an increase in the photocurrent resulting in improved solar cell efficiencies, which corresponds to an increase in the hole mobility in the polymer. We explain this enhancement in the solar cell performance in terms of decreased carrier recombination.