Influence of deposition rate on hole transport layer morphology and its effect on the electroluminescence stability of phosphorescent organic light emitting devices

Abstract

Material deposition rate during device fabrication is known to influence film morphology. Using the archetypical phosphorescent organic light-emitting devices (PhOLED) hole transport material 4,4′-Bis(carbazol-9-yl)biphenyl, the influence of the deposition rate on hole transport layer (HTL) morphology and its effect on the electroluminescence stability of PhOLEDs is investigated. Photoluminescence measurements show that films deposited at lower deposition rates tend to have a shorter exciton lifetime and a higher exciton stability. Atomic force microscopy measurements reveal that films deposited at various deposition rates develop different morphological features over time. More notably, films deposited at lower deposition rates exhibit increased surface roughness and the presence of larger crystalline regions when compared to their counterparts deposited at the higher rates when tested a few days after film deposition, indicating that the lower deposition rate leads to increased structural order in film morphology. We also found that using a lower deposition rate during HTL fabrication can lead to an increase in the electroluminescence stability of PhOLEDs. The increased stability can be attributed to the higher morphological or structural order and shorter exciton lifetime in them, hence a decreased susceptibility to exciton-induced degradation. These findings show direct correlations between material deposition rate, HTL morphology and PhOLED electroluminescence performance.