We have demonstrated efficient hybrid solar cells based on lead sulfide (PbS) nanocrystals and a narrow band gap polymer, poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]py rrole-1,4-diyl}-alt-{[2,2'-(1,4-phenylene)bis-thiophene]-5,5'-diyl}], (PDPPTPT). An opportune mixing of the two materials led to the formation of an energetically favorable bulk hetero-junction with a broad spectral response. Using a basic device structure, we reached a power conversion efficiency of similar to 3%, which is one of the highest values reported for this class of solar cells. Photo-physical measurements carried out on the device provided insights into the working mechanism: the comparison between the time decay of the pristine polymer and the polymer PbS blend allows us to conclude that efficient charge transfer is taking place in this hybrid system.