High efficient small molecule OLED lighting devices coated in a vacuum process have a thickness in the sub-μm range. Therefore, particles with a size down to 0.3 to 1μm can be critical to the device performance, like efficacy, dark spot formation and high leakage current. For this reason a roll-to-roll optical inspection system was set up which fulfils the defect resolution requirement in the μm range. It consists of a winding unit (from Spanntec Wickelsysteme GmbH) with an integrated inspection system of CCD line scan cameras as well as a moveable optical microscope (from IsraVision AG). The system provides different inspection modes, all of them geared towards the quantification and identification of the defect types which are critical for yield. The defects negatively affect the OLED lifetime through a high leakage current and the formation of dark spots. After the OLED lamination, the devices are investigated with the xy moveable microscope integrated in the roll-to-roll optical inspection system – see figure 4.
The high leakage current on top emitting OLED devices compromises the lifetime tests, because of the so called sudden death issue. Therefore, it is important to reduce particle contamination and defects initiated by web handling on an industrial scale. Secondly, the general substrate quality and surface roughness can be improved if the defect level falls under a certain threshold to allow lower leakage currents. More detailed investigation using different substrates, OLED materials, encapsulation techniques and barrier foils will be necessary to enable further up-scaling and commercialization.
Fig. 4: Optical inspection of dark spot growth on encapsulated OLED devices.
About the authors:
Dr. Stefan Mogck is Group Manager R2R at the Center for Organic Materials and Electronic Devices Dresden (COMEDD) in the Fraunhofer Institute for Photonic Microsystems (IPMS) -www.ipms.fraunhofer.de
Claudia Lehmann and Tomasz Wanski are Process Engineers at Fraunhofer IPMS – COMEDD
Christian Rahnfeld is Project Manager for Business Development, and Dr. Christian May is Head of Business Unit Lighting and Photovoltaics at Fraunhofer IPMS – COMEDD-
Courtesy of EETimes Europe
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