Here we report on the improved performance of hybrid polymer light emitting diodes (HyPLEDs) upon inserting an ultra thin layer of partially reduced tungsten oxide (WO(2.5)) or molybdenum oxide (MoO(2.7)), deposited (by heating a W or Mo filament while hydrogen was flowing through the chamber) at the polymer/Al cathode interface. Improved current densities, luminances and efficiencies were achieved as a result of improved electron injection and transport at its interface with Al. The observed electron injection improvement is attributed to the lowering of the effective cathode interfacial barrier due to the occupation of gap states with electrons after partial reduction.