Analysis of interfacial charges in aged co-host emitting layer of organic light-emitting diodes using impedance spectroscopy

This study investigated the charge injection and accumulation characteristics of organic light-emitting diodes (OLEDs) generated by spontaneous orientation polarization (SOP) of organic molecules. SOP enhances the electrical characteristics of OLED devices, such as charge injection and transport, by adjusting the interfacial charge density between the organic layers within the device. An OLED device with a mixed co-host emitting layer was fabricated to examine the degradation effect of SOP after electrical aging. Impedance spectroscopy (IS) revealed the presence of interfacial charges originating from the SOP of small molecules within the electron transport layer (ETL), in both pristine and aged devices. Furthermore, the IS simulation results using equivalent circuits were successfully interpreted based on the electrical properties of each organic layer in the pristine and aged devices. The electrical properties of the ETL, which contained 1,3,5-tris(N-phenyl benzimidazole-2-yl)benzene (TPBi), were degraded, reducing hole accumulation and electron injection.