Then, the ZnO QDs-graphene composite was dispersed in dimethylformamide (DMF) solvent and spun coated on top of the NPD layer to form the active region of the device. After that, a thin layer of 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) with thickness around 30 nm was evaporated onto the top of the active region as an electron transport/hole blocking layer and annealed in air at 5 C for 30 min. Finally,aluminum contact with thickness of 150 nm was deposited on top of the device through e-beam evaporator.