Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap-assisted recombination rate when [6,6]-phenyl C-84 butyric acid methyl ester (PC84 BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymerpoly[N-9 ''-hepta-decanyl-2,7-carbazole-alt-5,5-(4', 7'di-2-thienyl-2', 1', 3'-benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]-phenyl C-61 butyric acid methyl ester (PC60BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley-Read-Hall recombination; the latter is dramatically enhanced by the addition of the PC84BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap-induced recombination loss.