Cesium carbonate (Cs₂CO₃) is known as an excellent electronic injectio n material.To optimize its thickness,the devices of ITO/MoO₃(3nm)/NPB(40nm)/C545T:Alq₃/( 99∶1,0nm)/Alq₃(30nm)/Cs₂CO₃(x nm)/Al(100nm) were prepared in this work.The enhancement of electron injection was observed owing to the introduction of Cs₂CO₃EIL.Thus,more electrons recombine with holes in the emissive layer.The experimental results show that t he optimal thickness of Cs₂CO_3as electron injection layer (EIL) is 1.5nm.The effici ency of the 1.5-nm-Cs₂CO₃-based-device is more than three times of that of the dev ice without Cs₂CO₃.On the basis of it,the effect of Cs₂CO_3doped Bphen (Cs₂CO₃:Bphen) on the device performance is also investigated by varying Cs₂CO_{3concentration}.The device structures are ITO /NPB(40nm)/Alq₃(45nm)/ Cs₂CO₃:Bphen(0%,5%,10%,15nm)/ Cs₂CO₃(1.5nm)/Al(100nm).Consi dering the current density-voltage (J-V) characteristics of the electron -only devices of ITO/BCP(15nm)/Alq₃(45nm)/ Cs₂CO₃:Bphen(0%,5%,10%,15nm)/Cs₂CO₃(1.5nm)/Al,the following result s can be concluded:When the doping concentration is lower (5%),the electronic injection and transport c apacity of the device will be further enhanced,resulting in the higher luminous efficiency.Wh en the doping concentration is high (10%),the high efficiency roll-off is observed owning t o Cs atom quenching center induced by interdiffusion.