Analysis of Interfacial Charges by Spontaneous Orientation Polarization in the Co-Host EML Organic Light-Emitting Diodes with Impedance Spectroscopy

Organic light-emitting diodes (OLED) consist of a multi-layer such as hole injection layer (HIL), hole transport layer (HTL), emission layer (EML), electron transport layer (ETL), and electron injection layer (EIL) between two electrodes. By applying an external voltage, electrons and holes are injected and transported from the respective electrodes, resulting in light emission in the EML. In OLED, the presence of interfacial charges at the organic interface allows charge injection before applying voltage to the device. This property can be originated from the permanent dipole moment (PDM) of organic molecules. Most organic molecules used as electron transport layer, such as 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and tris-(8-hydroxyquinoline)aluminum (Alq₃), and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PyMPM) possess PDM, aligned spontaneously during the vacuum deposition process. The non-vanishing net aligned PDM, known as spontaneous orientation polarization (SOP), induces interfacial charges and affects charge injection properties. When the electron transport material of ETL forms a positive SOP against the cathode, the ETL shows negative interfacial charge (NIC) at the EML/ETL interface, influencing hole injection. Consequently, holes are injected and accumulated before the applying voltage to the device, and it can be shown in the capacitance-voltage (C-V) characteristics as the transition voltage (V_t). However, holes injected by NIC can lead to exciton-polaron quenching (EPQ) within the emissive layer. Therefore, it is important to understand the interface characteristics where holes accumulated by NIC. In this presentation, we investigated the difference in charge accumulation property caused by SOP at the ETL/EML interface in co-host EML OLED using impedance spectroscopy (IS). IS was employed to analyze the accumulation and transportation of charges in the co-host EML OLED.