Hole‐Trapping Effect of the Aliphatic‐Amine Based Electron Injection Materials in the Operation of OLEDs to Facilitate the Electron Injection

The organic materials containing aliphatic amine groups (AAGs) are widely used as the electron injection layer (EIL) in the application of organic light‐emitting diodes (OLED) to significantly improve the device performance. The electron injection mechanism is commonly explained by the interface dipole model, which suggests that the dipole layer induced by the electron injection layer at the organic/metal interface elevates the vacuum level on the metal side, thereby reducing the electron injection barrier. The experiments supporting the interface dipole model are generally carried out under the nonoperating condition of the OLEDs in which few holes are present. Our experiments on the OLED devices with neutralized AAG‐EIL dipole reveal that the interface dipole model cannot fully explain the AAG‐EIL's electron injection ability. During the operation of the OLEDs, the aliphatic amine function groups of the AAG‐EIL trap a huge amount of holes, bending the energy level and decreasing the electron tunneling distance. As a result, the reduced electron tunneling distance facilitates the electron injection.