Performance and lifetime of vacuum deposited organic light-emitting diodes: Influence of residual gases present during device fabrication

Understanding the influence of residual gases present during vacuum deposition of organic light-emitting diodes (OLEDs) and their effect on the device lifetime and the electrical characteristics of OLEDs is crucial for advancing industrial fabrication. In order to gain a more in-depth understanding, the influence of residual nitrogen, oxygen, and water vapor on lifetime and electrical characteristics is investigated. This is achieved by introducing the residual gases into the evaporation chamber through a needle valve while monitoring the partial pressures with the help of a mass spectrometer. We find that water vapor introduces a series resistance to the OLED while the other gases do not influence the electric characteristics. The presence of oxygen or nitrogen impacts the lifetime of the OLEDs by the same amount. Water vapor introduces an additional, even faster degradation process within the first hours of OLED operation. The electrically stressed OLEDs are analyzed by laser desorption/ionization time-of-flight mass spectroscopy. We identify the dimerisation of BPhen as well as the complexation reaction of a-NPD with an Ir(piq)2 fragment as sources of device degradation.