Electron injection and transport in 8-hydroxyquinoline aluminum

We have measured the current–voltage characteristics and device efficiency of organic light emitting diodes (OLEDs) based on 8-hydroxyquinoline aluminum (Alq3) in combination with several cathode layer setups. The electron injection properties of cathode metals evaporated under high vacuum (HV) and ultra-high vacuum (UHV) conditions are compared. Further, cathodes incorporating a thin layer of lithium fluoride, which is covered with a metal capping layer, are investigated. It will be shown that aluminum is an outstanding capping metal and significantly improves both electron injection and device efficiency. Quasi-static and transient current–voltage measurements on single-layer devices will be presented. It will be demonstrated that cathodes, comprising 0.2 nm LiF and aluminum, are able to sustain space charge limited currents in Alq3. Additionally, the efficiency and lifetime data of multi-layer devices using this cathode layer setup are discussed.