Candidate
Qi Wang
Title
Exciton-Induced Degradation of Interfaces in Organic Light-Emitting Devices
Supervisor
Hany Aziz
Abstract
Organic light-emitting devices (OLEDs) are emerging as the next generation technology for flat panel displays and solid-state lighting because of their advantages in realizing low-cost, large-size and mechanically flexible applications. A primary barrier for PhOLEDs commercialization, however, is their extremely short lifetime. Previous studies on PhOLEDs degradation mainly focused on degradation phenomena that occur in the organic layers bulk. The possibility that degradation may be primarily interfacial was however overlooked.
We recently determined that organic/organic and organic/electrode interfaces degrade significantly due to the presence of excitons in their vicinity. In the case of organic/electrode interfaces, the exciton-induced degradation reduces both charge injection and charge extraction. X-ray photoelectron spectroscopy (XPS) measurements reveal detectable changes in the interface characteristics after degradation, indicating that the process is chemical in nature. In the case of organic/organic interfaces, the degradation is found to be closely linked to interactions between excitons and positive polarons in the host material which leads to its aggregation. Quite notably, such aggregation appears to correlate with the materials bandgap rather than with their glass-transition temperatures.
These results shed the light on a new material degradation mechanism that appears to have a wide presence in organic/organic and organic/metal interfaces in general, and likely plays a key role in limiting the stability of various organic optoelectronic devices.