Temperature and environmental stability studies of OLED's based on small molecules

We investigated environmental and temperature stability of organic electroluminescent devices based on small molecules deposited by the vacuum evaporation method. As model systems OLED's with the following structures were used: ITO/TPD/AlQ3/MgAg and ITO/CuPc/NPB/AlQ3/MgAg. The presence of water is well known to cause a rapid degradation of OLED's by formation of black spots. In TPD based cells we found evidence for TPD crystallization in the black spot areas. The presence of oxygen, on the other hand, is not found to significantly impact device life. In TPD devices rapid degradation is observed already at 40 °C which also effects device shelf life. NPB based devices are much more stable. Accelerated aging at high currents (>30 mA/cm²) showed a superlinear dependence of cell life on the reciprocal driving current. To separate thermal and current induced degradation, cells were cooled during high current operation and maintained at room temperature. Accelerated aging results for different cell temperatures will also be discussed.