Sputter Depth-Profile Study of Accelerated Interface Mixing by Thermal Annealing in Solution-Processed Organic Light-Emitting Diodes

The effect of thermal annealing on interfacial mixing of solution-processed organic light-emitting diodes (OLEDs) using direct sputter-depth profiling techniques is investigated. X-ray and ultraviolet photoelectron spectroscopy and argon gas cluster ion beam sputtering are used to investigate the distribution of chemical species near the interface. Extensive interfacial mixing is found in solution-processed OLEDs after the thermal annealing at temperatures below the glass transition temperature of the organic material, while such mixing is not evident in vapor-deposited devices. It is found that there is a partially mixed interface in the solution-processed devices prior to the annealing, which seems to promote effective interdiffusion during the annealing. Surprisingly, the extensive interfacial mixing after the thermal annealing appears to significantly enhance the efficiency of OLEDs.