We show how molecular doping can be implemented to improve the performance of solution processed bulk heterojunction solar cells based on a low-bandgap polymer mixed with a fullerene derivative. The molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) is introduced into blends of poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]- dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) via co-solution in a range of concentrations from 0% to 1%. We demonstrate that the hole conductivity and mobility increase with doping concentration using field-effect measurements. Photoinduced absorption (PIA) spectroscopy reveals that the polaron density in the blends increases with doping. We show that the open circuit voltage and short circuit current of the corresponding solar cells can be improved by doping at 0.5%, resulting in improved power conversion efficiencies. The increase in performance is discussed in terms of trap filling due to the increased carrier density, and reduced recombination correlated to the improvement in mobility.