Degradation Behaviors of NPB Molecules upon Prolonged Exposure to Various Thermal Stresses under High Vacuum below 10–4 Pa

Understanding of the long-term thermal stabilities of organic light-emitting diode (OLED) materials during film deposition is important to accurately identifying their processing windows. The thermal stresses imposed on OLED materials in the evaporation source during the deposition process may cause phase transition and/or degradation of the source materials, which results in variations in their purity and thermal properties, such as the vapor pressure and, ultimately, the device degradation. In this work, we designed a simple and efficient apparatus to determine the long-term thermal stability of OLED materials, which allows prolonged heating of a minimal amount of the sample (∼2 g) for 50 h even under high vacuum below 10 -4 Pa where the organic powder samples easily and rapidly vaporized because of exposure to temperature above their deposition temperature. We used this apparatus to evaluate the thermal degradation behaviors of N , N '-bis(1-naphthyl)- N , N '-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB), which is a widely used hole-transporting material in OLEDs, upon prolonged exposure to various thermal stresses. After prolonged heating at 330 °C (380 °C) for 25 h (50 h), the change in purity, mass, vapor pressure, and phase of the heated NPB were analyzed by high-performance liquid chromatography, liquid chromatography-mass spectrometry, thermogravimetric analysis, and X-ray diffraction. The lifetime of OLEDs using the heated NPB was measured to study how the thermally induced material degradation affects the device characteristics. The results showed that the NPB degradation caused by prolonged exposure to 330 °C accelerated over time. In addition, it was confirmed that the degradation products with high molecular weight that form due to exposure to 380 °C was the main cause of device degradation.