Enhanced electron injection in organic light-emitting devices by using a composite electron injection layer composed of 8-hydroquinolatolithium and cesium oxide

A novel composite electron injection layer (cc-EIL) composed of 8-hydroquinolatolithium (Liq) and cesium oxide (CsOx) was sandwiched between tris(8-hydroquinoline) aluminum (Alq3) and an Al cathode to enhance electron injection in organic light-emitting devices (OLEDs). Using this strategy, the luminous efficiency was enhanced by ∼22% and ∼31% at the current density of 20 mA/cm2 and 200 mA/cm2, respectively, and the power efficiency has been increased by ∼29% and ∼36% at the same current densities, as compared with the control device when the optimal composition of Liq(1 nm)/CsOx(1 nm) was employed. The enhancement of electron injection by incorporating cc-EIL was further verified by electron-only devices and was explained by the dipole effect and a step barrier provided by the intermediate energy level of Liq interlayer. Our results strongly suggested that the cc-EIL was an excellent choice to enhance electron injection and would surely contribute to stability of OLEDs since a better improvement in luminous and power efficiencies at higher current density was observed.