Exciton-Induced Morphological Changes in Carbazole Based PhOLEDs and the Influence of the Device Fabrication Process

Abstract

Organic light-emitting devices (OLEDs) have increasingly attracted attention from both academia and industry alike in the last three decades because of their low cost, easy fabrication, light weight, and mechanical flexibility; a unique set of features that makes them particularly well-suited for utilization in flat panel displays and solid-state lighting. In general, display and lighting applications demand high efficiency and long lifetime OLEDs, characteristics that strongly depend on the photophysical properties of the materials involved. While those properties are predominantly governed by material chemistry and molecular structure, they are also influenced by intermolecular processes and thus by molecular packing and morphology in the solid state. A better understanding of the dependence and interplay between photophysical properties and morphology, especially with regards to the effects of exciton stress on morphology in the context of device performance, therefore, becomes critical. This is because while excitons are prerequisites for light emission in OLEDs, they are also one of the main degradation agents in these devices as the excitation energy that molecules have in this state can drive various chemical and physical aging processes. This work studies the effects of exciton-stress in organic small molecule electroluminescent materials with the specific purpose of investigating two aspects: (i) the effect of exciton stress on material morphology and the subsequent effect of that on device performance; and (ii) the dependence of exciton-induced aging on device fabrication processes. Because of its prevalent use in phosphorescent OLEDs (PhOLEDs), the carbazole material system is used for these investigations. First, exciton-stress has been found to lead to increased surface roughness in thin films of organic electroluminescent material, representing the first direct evidence of exciton-induced aggregation in these systems. Results show that exciton stress leads to the formation of molecular aggregates that can then act as nucleation sites for subsequent crystal growth. Next, exciton-stress in both the hole transport layer (HTL) and electron transport layer (ETL) of PhOLEDs has been found to have a detrimental effect on device performance revealing that exciton-induced degradation in these devices may not be limited to the light emitting layer of the devices as was believed before, but rather extend to the charge transport layers. Interactions between the ETL materials and exciton-induced host molecular aggregates are found to produce complex species, a phenomenon that underlies the red shift often observed in the electroluminescent spectra of carbazole-based PhOLEDs over time. Similarly, exciton stress in HTL also affects device performance, in this case through the formation of quenchers that degrade efficiency and stability. Reducing exciton-induced degradation of the HTL is found to be crucial for improving device efficiency and stability. Comparative investigations of solution-coated versus vacuum-deposited OLEDs shows that solution-coated materials have a greater susceptibility to exciton-induced degradation. They are also found to have a lower host-to-guest energy transfer efficiency, an effect that likely originates in differences in morphology. Finally, exposure of PEDOT:PSS, the hole injection material most widely used in solution-coated OLEDs, to solvents during the HTL coating process has been found to degrade the hole injection efficiency of the contact. This phenomenon has been found to play a major role in the lower electroluminescence stability of solution-coated OLEDs.