Thermally induced failure mechanisms of organic light emitting device structures probed by X-ray specular reflectivity

The thermal evolution of organic light emitting device structures and materials has been measured using X-ray specular reflectivity. Thermally induced failure of these structures is found to be due to the large thermal expansion of the hole transport layer (N,N′-diphenyl-N,N′-bis(3-methylphenyl)1,1′-biphenyl-4,4′diamine, or TPD) associated with its low glass transition temperature, suggesting a strain-driven failure mechanism (as opposed to TPD recrystallization). These results suggest the possibility of optimizing organic light emitting devices through direct in-situ measurements of their structure and stability.