The brightness of organic light-emitting diodes, or OLEDs, declines over time. Why exactly the lights degrade hasn't been entirely clear to scientists -- until now.

Researchers at Kyushu University in Japan determined impurities in the vacuum chamber during the OLED fabrication process explain degradation. The impurities are so minuscule they're often overlooked.

OLEDs are made up of a stack of organic layers. The layers, which convert electricity into light, are heated and deposited onto a substrate in a vacuum chamber.

"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," says lead author Hiroshi Fujimoto, a visiting associate professor at Kyushu.

During a series of experiments, scientists found OLED lifetimes -- the amount of operational time before degradation and dimming -- corresponded with the amount of time layers spent in the deposition chamber. The longer the time spent in the chamber, the more time the stacked layers had to accumulate impurities.

"Really small amounts of these impurities get incorporated into the fabricated devices and are causing large changes in the lifetime," explained Chihaya Adachi, director of Kyushu's Center for Organic Photonics and Electronics Research.

The research, published in the journal Scientific Reports, is a reminder of the importance of cleaning vacuum chambers and minimizing exposure time during OLED fabrication.

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