Dependence of the efficiency improvement of organic light-emitting diodes on the thickness of the Cs2CO3 electron-injection layer

An efficiency improvement of organic light-emitting diodes (OLEDs) was studied by varying the thickness of the electron-injection layer (EIL). First, an optimum EIL thickness of Cs2CO3 was confirmed by using a simulation program. Then, the OLEDs were designed to have a structure of indium-tin-oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/tris(8-hydroxyquinolinate) aluminum (Alq3)/cesium carbonate (Cs2CO3)/aluminum (Al). By using the thermal evaporation method, we manufactured specimens to the optimum thickness of the material found in the simulation, and we investigated how the Cs2CO3 EIL affected the efficiency of the OLEDs. As a result, because the Cs2CO3 EIL reduced the potential barrier in the cathode by 0.08 eV, it facilitated the movement of electrons, which confirmed that the Cs2CO3 improved the efficiency of the OLEDs as it increased recombination by blocking hole movement. When compared to the device without the EIL, the device with a 1.0 nm-thick Cs2CO3 EIL showed an excellent efficiency. The luminance and the external quantum efficiency increased about 600% and 500%, respectively.