Confinement of Charge Carriers and Exdtons in Electrophosphorescent Devices: Mechanism of Light Emission and Degradation

摘要

The light-emission mechanism and lifetime of an organic light-emitting device (OLED) are determined by the properties of the organic material in the devices. In OLEDs, the emission mechanism and the phenomena that degrade emission are affected by various factors, including the energy levels of the organic material, the charge-carrier trapping and transport characteristics, recombination in the light-emitting layer (EML), and their interplay. Moreover, in OLEDs based on phosphorescent organic materials, the long-life excited states and higher luminous quantum yields of the phosphorescent material mean that different mechanisms, such as preferentially direct charge-carrier trapping and exciton migration, dominate the process of light emission and recombination. However, the doping and charge-trapping behavior of phosphorescent organic materials and their multilayers have yet to be thoroughly studied. In multilayered devices, transport and blocking layers are used to confine excitons that are originally formed in the dye-doped emission layer, enabling efficient emission to be maintained. Therefore, both for small-molecule and polymeric-type phosphorescent organic materials, the OLED structures used for charge balancing and exciton confinement are important for stable device operation and higher efficiency, because the properties of these devices strongly depend on the interactions between the layer of phosphorescent material and adjoining layers. Greater knowledge of the physics underlying the operation of multi-layered OLEDs is essential for the commercial development of highly efficient large-area OLEDs with long lifetimes.