To investigate the effect of the thickness of electron transport layer (PO-T2T) on the luminescent properties of TPD:PO-T2T yellow exciplex,the devices with a structure of ITO/HAT-CN/TPD/TPD:PO-T2T/PO-T2T (x=10,20,30,40,50,60,70 nm)/LiF/Al are fabricated by multi-source organic molecular vapor deposition system (LN-386SA).The electroluminescent (EL) spectra of the devices are barely changed by the thickness of PO-T2T,while other properties such as current density (CD),luminance as well as efficiencies are greatly changed.The luminescent efficiencies are increased with the increase of PO-T2T thickness,which is mainly attributed to the reduction of quenching center in the emissive layer caused by the diffusion of aluminum cathode.When the thickness of PO-T2T is 70 nm,the maximums of current efficiency (CE) and power efficiency (PE) are 2.16 cd/A and 2.12 lm/W,respectively.In addition,transient overshoot and recombination of carriers captured by deep traps are not observed through the transient EL properties of the devices,which indicates that the emission of the TPD:PO-T2T exciplex are originated from the direct carrier recombination.