The role of OLED emissive layer polarization in sub-turn-on charge accumulation

Abstract

Preferential alignment of molecular permanent dipole moments, known as spontaneous orientation polarization (SOP), is present in many materials employed in the active layers of organic light-emitting devices (OLEDs). This phenomenon leads to the formation of bound polarization charge, which is compensated by polaron accumulation at voltages below turn-on. While most prior work has focused on polarization in the device electron transport layer (ETL), here we examine the impact of emissive layer SOP by systematically probing polaron accumulation and exciton-polaron quenching in phosphorescent OLEDs. To gain a deeper understanding of polaron accumulation, device capacitance is systematically probed as a function of voltage across samples with polar and nonpolar emissive layers. We find that capacitance measurements can be used to track not only the number of accumulated charges, but also its location within the device active layers. This study provides an analysis framework that allows further insights on the charge accumulation process in OLEDs, thus improving our understanding of SOP in OLEDs