Vacuum Impurities Could Impact OLED Lifetime

While issues affecting the efficiency of OLEDs are well understood, the reasons why some OLEDs degrade and lose brightness over time have not been as easy to identify. Devices fabricated with seemingly the same procedures and conditions but by different research groups often degrade at vastly different rates, even when the initial performances are the same. One possible explanation for discrepancies in OLED lifetimes could be miniscule impurities present in the vacuum chamber during fabrication.

Researchers from Kyushu University investigated the influence of impurities in a vacuum chamber used for the fabrication of OLEDs. Their analysis revealed that tiny amounts of impurities present in the vacuum chamber during OLED fabrication were potentially being incorporated into OLEDs, leading to variations in lifetime. They also found a correlation between lifetime of the OLED and device fabrication time.

Research shows that miniscule amounts of impurities in the vacuum are being incorporated into OLEDs during fabrication and leading to large variations in lifetime. By reducing the time OLEDs spend in the deposition chamber during fabrication, impurities can be reduced and lifetime enhanced. Analysis of the impurities indicates sources that include previously deposited materials and plasticizers from chamber components. Courtesy of Hiroshi Fujimoto and William J. Potscavage Jr.

Using liquid chromatography-mass spectrometry on silicon wafers stored in the vacuum chamber before device fabrication, the researchers detected a variety of materials and found that many of the impurities could be traced to previously deposited materials and plasticizers from the vacuum chamber components.

The researchers found that, because of impurities in the deposition chamber, lifetime sharply increased for OLEDs that spent a shorter time in the deposition chamber during fabrication. This trend remained even after considering changes in residual water and source material purity, indicating that controlling and minimizing the device fabrication time could lead to longer lifetime for OLEDs.

Research partners at Sumika Chemical Analysis Service Ltd. (SCAS) confirmed an increase of accumulated impurities with time by analyzing the materials that deposited on extremely clean silicon wafers that were stored in the deposition chamber when OLED materials were not being evaporated.

Results suggest that the impurities floating in a vacuum chamber could significantly impact OLED lifetime values and reproducibility, even if they amount to less than even a single molecular layer.

"Although we often idealize vacuums as being clean environments, we detected many impurities floating in the vacuum even when the deposition chamber is at room temperature," said professor Hiroshi Fujimoto.

"Really small amounts of these impurities get incorporated into the fabricated devices and are causing large changes in the lifetime," professor Chihaya Adachi said.

To improve lifetime reproducibility, a practice often adopted in industry is the use of dedicated deposition chambers for specific materials, but this can be difficult in academic labs, where often only a limited number of deposition systems are available for testing a wide variety of new materials. In these cases, deposition chamber design and cleaning in addition to control of the deposition time are especially important.

"This is an excellent reminder of just how careful we need to be to do good, reproducible science," said Adachi.

The research was published in Scientific Reports (doi:10.1038/srep38482).