Long-term thermal stability of NPB molecule under high-vacuum

Abstract During the deposition of organic light emitting diodes (OLEDs) using thermal evaporation, the purity of the deposition-source materials varies due to thermal stress, resulting in a reduced device stability. We developed an apparatus to evaluate the long-term thermal stability of deposition-source materials used in OLEDs, which is applicable even under high-vacuum conditions (below 10−4 Pa) where the materials under test run out very quickly when exposed to temperature greater than 300 °C, which is above their deposition temperature. The goal of this work is to understand how thermal stresses imposed on OLED materials for a certain duration affect the material purity and, ultimately, the device performance. We used this apparatus to expose N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), a commonly used hole-transport material for OLEDs, to thermal stresses for various durations. After maintaining the samples at 330 °C (380 °C) for 25 h (50 h), we performed high-performance liquid chromatography to analyze the material purity. In addition, OLEDs with a simple bilayer structure (NPB/tris(8-hydroxyquinoline)aluminum, Alq3) were fabricated from the heated samples to investigate how material impurities that are induced by thermal stresses affect the device stability. The results show that slight changes in the NPB purity caused by prolonged exposure to high temperatures significantly influence the electrical and optical characteristics of OLEDs fabricated from the heated NPB.