We report a significant improvement in the performance of single layer polymer light-emitting diodes (PLEDs), based on the green emitting copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2, 1^′ ,3}-thiadiazole)], upon inserting a very thin layer of partially reduced molybdenum oxide ( MoO _x , where x =2.7 ) at the polymer/Al cathode interface. Both fully oxidized ( x =3) and partially reduced ( x =2.7) thin molybdenum oxide layers were investigated as electron injection layers and their influence on PLED device performance was examined. Improved current density, luminance, and efficiency was achieved only in the case of devices with a thin partially reduced MoO _2.7 film as electron injection layer, as a result of improved electron injection and more facile transfer at the modified polymer/Al interface.