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Roll-to-roll processing for small molecule flexible OLED devices

Posted: 24 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:roll-to-roll? passivation layer? optical inspection?

Recently, a number of lighting companies in Europe and Asia have begun pilot production of OLED devices on glass. The OLED covers a range of applications from less demanding signage and decorative lighting up to large area flexible illumination, automotive applications and general lighting which have higher requirements in terms of efficiency and reliability. It is believed that the price can drop significantly by transferring the sheet-to-sheet fabrication into the roll-to-roll (R2R) processing. High throughput and the use of relatively cheap metal foil and plastic web as substrates will be the major cost reducing step. Aluminum foil already fulfils one important requirement as a substrate for OLEDs regarding impenetrability to water and oxygen.

Still a challenge is the flexible encapsulation, allowing transparency while offering sufficient barrier properties against water and oxygen, together with mechanical stability. One of the limitations for OLED lifetime and therefore for OLEDs is the water vapor transmission rate (WVTR), which should be kept below 10-6 g/d*m?. A common approach to fabricate barrier films is to use multilayer stacks. At present semi-commercial barrier films on rolls (large area coatings) are available which could already meet the requirements for lower demanding applications.

To carry out research and develop flexible OLED lighting modules, the "Center of Organic Materials and Electronic Devices Dresden" (COMEDD) of Fraunhofer Institute for Photonic Microsystems (IPMS) has set up a roll-to-roll line (R2R). It consists of a vacuum coater for small molecule deposition, a R2R encapsulation unit operated under inert atmosphere and a R2R optical inspection system for defect characterization. The encapsulation concept includes a thin film pre-encapsulation followed by a barrier film lamination. A mobile roll transfer box allows an inert roll transfer between the vacuum coater and the lamination unit to keep the overall H2O and O2 concentration in the protective atmosphere below 10ppm, respectively.

Figure 1: Schematic drawing of the R2R vacuum coater which includes the winding unit, plasma pre-treatment with a linear ion source, organic linear evaporators, a magnetron and metal evaporators.

Roll-to-roll OLED deposition, encapsulation
R2R vacuum deposition and fabrication of small molecule OLEDs on flexible substrates are carried out in the RC 300-MB roll-to-roll vacuum coater (supplied by Von Ardenne Anlagentechnik GmbH). The machine can process metal or plastic substrates with a width of 300mm and a thickness of 70 to 500?m. After OLED deposition, the substrate can be enveloped by a protective liner film (interleaf liner) to minimize the surface damage of the sensitive organic layers, as schematically shown in figure 1. After the vacuum coating, the process chamber can be vented with nitrogen to keep the substrate protected against moisture and oxygen. Before the OLED process can be started, a non-conductive passivation layer must be printed on the aluminum band (figure 2). The substrate patterning with a passivation layer is necessary to isolate the anode (aluminum foil) from the metallization (cathode and metal contact lines) after the OLED stack deposition. During the organic- and metal evaporation a shadowing with integrated strip masks allow a proper patterning of devices. The OLED devices were tested in the encapsulation unit after the lamination with the barrier film (figure 3).

Figure 2: Patterning of the homogeneous metal foil with a nonconductive white passivation layer.

Figure 3: Testing a white pin OLED on an aluminum band in the encapsulation unit after the OLED encapsulation with a barrier foil.

Roll-to-roll inspection system, defect recognition
High efficient small molecule OLED lighting devices coated in a vacuum process have a thickness in the sub-?m range. Therefore, particles with a size down to 0.3 to 1?m 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?m 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 toward 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 (figure 4).

Figure 4: Optical inspection of dark spot growth on encapsulated OLED devices.

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.

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).

Claudia Lehmann and Tomasz Waski are Process Engineers at Fraunhofer IPMS C COMEDD.

Christian Rahnfeld is Project Manager for Business Development, and Dr. Christian May is Head of Business Unit Lighting and Photovoltaics at Fraunhofer IPMS C COMEDD.

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