Effects of Charge Dynamics in the Emission Layer on the Operational Lifetimes of Blue Phosphorescent Organic Light-Emitting Diodes

Abstract

© 2022 Wiley-VCH GmbHDespite more than 20 years of research, the root cause of the impractically short lifetimes of blue phosphorescent organic light-emitting diodes (PHOLEDs) has remained unclear. To overcome this, the authors investigate how the electrical properties of the emission layer (EML) of blue PHOLEDs affect degradation of the devices. It is found that a large density of dopant carriers is the dominant factor triggering triplet-polaron annihilation (TPA), which is a major defect-generation and hence lifetime-reduction mechanism. In order to reduce the generation of the TPA-induced defects to ensure long device lifetimes, the dopant carrier density should be minimized by suppressing the spontaneous charge transfer from the host to the dopant initially and by supplying sufficient charges with opposite polarity into the EML. However, there exists another critical factor that offsets the low overall density of defects against device lifetimes—that is, the non-uniform distribution of defects leading to intense exciton quenching. These two degradation factors are predetermined, and hence can be controlled, by the charge mobilities of the PHOLED EML. Given these considerations, it is demonstrated that the long-lifetime blue PHOLEDs can be realized.