Miniscule Amounts of Impurities in Vacuum Greatly Affect OLED Lifetime

organic light-emitting diodes
Top left: Green OLEDs undergoing lifetime tests Bottom left: Dependence of OLED lifetime on the length of time the device spent in the deposition chamber during fabrication Right: Conceptual image illustrating some of the sources of impurities in the deposition chamber that are affecting OLED lifetime

Reproducibility is a necessity for science and manufacturing but has long eluded researchers studying the lifetime of organic light-emitting diodes (OLEDs). New research sheds light on the cause: impurities present in the vacuum chamber during the fabrication of OLEDs but in amounts so small that they are easily overlooked. Because of this, researchers found that the time until an OLED under operation dims by a given amount because of degradation, known as the lifetime, sharply increases when the time an OLED spends in the vacuum chamber during fabrication is reduced. Although vacuums are often idealized as being clean environments, unwanted impurities in the vacuum system contaminate the OLED materials being thermally evaporated to make the active layers of the device and become an important factor affecting device degradation. Using liquid chromatography-mass spectrometry to identify the impurities, the researchers found that many could be traced to previously deposited materials and plasticizers from the vacuum chamber components. While control of water content has been known to be important for improving lifetime, these new results show that impurities amounting to less than even a single molecular layer from sources such as the fabrication chamber itself must be closely managed to reproducibly achieve highly stable and reliable devices.

This work was performed by the research group led by Prof. Chihaya Adachi from Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Visiting Associate Prof. Hiroshi Fujimoto from Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA), and Visiting Prof. Masayuki Yahiro from Institute of System, Information Technology and Nanotechnology (ISIT) in cooperation with Sumika Chemical Analysis Services Ltd.