Poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2′-thienyl-2,1,3-benzothiadiazole)]}:PCBM bulk heterojunction solar cells were fabricated and characterized under different incident light power intensities. Charge-trapping effects take place at low fullerene content in the photoactive blend; an efficient polymer fullerene intermixing with formation of continuous phases is reached at a donor:acceptor ratio of 1:4. For an optimized active layer thickness of 100 nm, a power-conversion efficiency of 2.57% was obtained. Photocurrent measurements under reverse-bias conditions show that a high percentage of the photogenerated excitons does not lead to the formation of free carriers, thus representing the major limiting factor for the device’s efficiency.