Temperature-independent electron tunneling injection in tris (8-hydroxyquinoline) aluminum thin film from high-work-function gold electrode

We fabricated electron-only tris (8-hydroxyquinoline) aluminum (Alq₃) single-layer devices with a device structure of glass substrate/MgAg anode (100 nm)/Alq₃ layer (100 nm)/metal cathode (100 nm), and systematically varied the work functions (WF) of the metal cathodes from WF = - 1.9 (Cs) to - 2.9 (Ca), - 3.8 (Mg), - 4.4 (Al), - 4.6 (Ag), and - 5.2 eV (Au) to investigate how electron injection barriers at the cathode/Alq₃ interfaces influence their current density-voltage (J-V) characteristics. We found that current densities at a certain driving voltage decrease and the temperature dependence of J-V characteristics of the devices gradually becomes weaker as the work functions of the metal cathodes are decreased. The device with the highest-work-function Au cathode exhibited virtually temperature-independent J-V characteristics, suggesting that a current flow mechanism of this device is mainly controlled by electron tunneling injection at the Au/Alq₃ interface.