Recently, several lighting companies in Europe and Asia have started 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 multi-layer 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 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.
Roll-to-roll OLED deposition and 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 500µm. 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 – see 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 - see figure 3.
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.
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.