Root Causes of the Limited Electroluminescence Stability of Organic Light-Emitting Devices Made by Solution-Coating

Although organic electroluminescent materials have long promised the prospect of making organic light emitting devices (OLEDs) via low-cost solution-coating techniques, the electroluminescence stability of devices made by such techniques continues to be rather limited making them unsuitable for commercialization. The root causes of the lower stability of OLEDs made by solution-coating versus the more conventional vacuum-deposition remain unknown. In this work, we investigate and compare between solution-coated and vacuum-deposited materials under prolonged excitation, using the archetypical host material 4,4'-bis( N-carbazolyl)-1,1'-biphenyl as a model OLED material. Results show that solution-coated films are more susceptible to degradation by excitons in comparison to their vacuum-deposited counterparts, resulting in a faster decrease in their luminescent quantum yield. The degradation rate also depends on the choice of solvent that was used in the solution-coating process. Results also show that the decrease in quantum yield is caused by exciton-induced chemical decomposition in the material as well as some possible molecular reorganization or aggregation, both of which are induced by excitons and proceed more quickly in case of solution-coated films. The faster degradation in the solution-coated films appears to originate primarily from their different morphological makeup and not due to chemical impurities. The findings uncover what appears to be one of the fundamental root causes of the lower stability of solution-coated OLEDs in general.